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introduction- discs
- drums
- master cylinder
- bleeding
- f.a.q.
HEL Performance Brake Doctor - Brake Fade
Introduction
Pad Fade
Green Fade
Fluid Fade
Reducing Pad Fade
Reducing Green Fade
Reducing Fluid Fade
Brake fade can be the number one braking problem that is encountered on road or track. It is rare on road as the limits of the braking components are not exceeded but the following information may be of interest to road racers, track day enthusiasts and racers.
Brake fade occurs after braking hard several times in a row, for example during road racing or on track, and you lose braking effectiveness. This usually occurs gradually so you can compensate in your brake point by braking sooner, but sometimes happens so suddenly you can end up going on a wild off-road excursion with sometimes painful results.
There are three kinds of fade commonly encountered in fast riding; pad fade, green fade and fluid fade.
Brake Doctor - Pad Fade
Pad fade occurs for several reasons. All friction material (the materials brake pads are made of) has a coefficient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is the highest. Sometimes you can use the brakes so hard that you get the temperature over the point of maximum friction to where the coefficient of friction curve starts to decline.
The mechanics of this decline in the coefficient of friction are varied. At a certain temperature, certain elements of the pad can melt or smear causing a lubrication effect, this is the classic glazed pad. Usually the organic binder resin starts to go first, then even the metallic elements of the friction material can start to melt. At really high temperatures the friction material starts to vaporize and the pad can slide on a layer of vaporized metal and friction material which acts like a lubricant. Pad fade is felt on a bike that still has a firm, 'non spongy' feeling brake lever that won’t stop even if you are squeezing as hard as you can. Usually it builds somewhat slowly giving you time to compensate for it but some friction materials have a sudden drop off of friction when the heat is put on them resulting in sudden dangerous fade. [Top]
Brake Doctor - Green Fade
This is perhaps the most dangerous type of fade. Green fade is a type of fade that manifests itself on brand new brake pads. Brake pads are usually made of different types of heat resistant materials bound together with a phenolic resin binder. These are thermosetting plastic resins with a high heat resistance. On a new brake pad, these resins will cure when used hard on their first few heat cycles. The new pad can hydroplane on this layer of excreted gas. Green fade is dangerous because many people assume that new brakes are perfect and can be used hard right off the bat. Green fade typically will occur much earlier than normal fade so it can catch a rider that is used to a certain bikes characteristics unaware. Typically the onset of green fade is rather sudden, further increasing the danger factor. Some teams have a new pads warning sticker that they place on the top yoke to inform the rider to be careful on the first few laps.
Green fade can be prevented by bedding in the pads correctly. This is a simple procedure to boil off the resins and break in the pads under controlled conditions which has been explained below - Reducing Green Fade [Top]
Brake Doctor - Fluid Fade
Fluid fade is caused by heat induced boiling of the brake fluid in the calipers. This produces bubbles in the brake system. Since bubbles are compressible, this makes for a soft spongy lever. In worse cases, the lever can come all the way back to the grip without slowing the bike.
The major cause of fluid fade is absorbed atmospheric water in the brake fluid. DOT4 Brake fluid has a tremendous affinity for absorbing water from the atmosphere, especially at high temperatures and under humid conditions. Brake fluid can absorb atmospheric water just by sitting in the brake system of your bike. A container of brake fluid sitting open can literally go bad from water absorption in an hour on a hot humid day. It is important to keep your fluid in a tightly sealed container, keeping the cap on at all times except when pouring fluid out!
Fluid fade can be avoided by running a higher grade racing type brake fluid and most importantly frequent changes of brake fluid. Regularly changing the fluid is the most important thing you can do to avoid fluid fade - change your brake fluid every 3 months and fit braided brake hoses and you will have superb brakes.
Fortunately fluid fade usually has a gradual onset, usually enabling you to either slow down or pit before a total loss of brakes occurs.
The reason we are explaining these forms of fade to you is so that you can identify what kind of fade that you are suffering from and do the proper thing to fix the type of fade that you have with the correct countermeasure. If you are experiencing pad fade, switching brands of brake fluid won’t help. If you are getting fluid fade, the trickiest carbon pads won’t stop you a bit sooner. If you have the finest brake parts available, you could still fall prey to green fade. [Top]
Brake Doctor - Reducing Pad Fade
Pad fade is easily reduced by getting pads with a higher coefficient of friction at higher temperatures. On most bikes the stock pads, genuine Nissin, are remarkably good but in our experience Ferodo Brake Pads (as used by most Isle Of Man TT competitors since 1924 and also by Duke, Agostini, Surtees, Sheene, Lawson, Dunlop, Fogarty...) have a great range of pads available for most motorcycles. They offer Platinum Organic, SinterGrip and Racing CP911 (Strictly for Track Use Only).
Brake Doctor - Brake pads can be roughly broken down into about 4 types:
Organic - (Ferodo : Platinum Pads) Made of stuff like cellulose, which is like ground up cardboard! The cellulose is held together with a phenolic resin binder which is a heat resistant thermosetting resin. Organic pads used to have asbestos to give better high temperature properties but since asbestos is now a carcinogenic, kevlar, fiberglass and mineral fillers are now also used. Organic pads have a good coefficient of friction for a light lever effort, work well at low temperatures and are very quiet. They are not as good for high performance use as they quickly wear, fade, oxidize and crumble. Organic pads are kind of old school and are common on cheap aftermarket replacement pads for older and sometimes new bikes. These pads do not wear the discs very much. Organic pads are usually a light brown or tan in color.
Semi-Metallic - These have some powdered metal added to the mix to help stabilize the coefficient of friction at higher temperatures. Typically powdered Brass, iron or Bronze is added. Chopped brass or bronze wire is sometimes added to help give the pad more mechanical strength. Usually these pads are excellent for all-around use. The more metal added usually means better high temperature properties, more noise, more rotor wear and less effective cold braking. Semi-Metallic can be light tan with metal flecks in them to a dark gray in color. The darker pads usually indicate a higher metal content. A higher metallic content, semi metallic is usually a good all around high performance street pad.
Full metallic - (Ferodo : SinterGrip Pads) These pads are made of sintered metal with very little binder. Sintered metal is powdered metal that is pressed into a mold at high temperatures until it becomes a more or less homogeneous piece. Pads of this type are pretty aggressive with ones made of brass, bronze or copper or a mix of metals being more for street use and ones using iron being more high temperature oriented. For very high temperature use, ceramic powder is added to the pad material. Full metal pads usually require more lever effort to stop the bike. These pads produce corrosive black brake dust so clean your rims frequently. These pads are usually a dark gray to black and sometimes even copper-looking with a lustrous sheen.
Carbon - Carbon pads available to us mere mortals are not the amorphous carbon-carbon exotica that F-1 cars, the space shuttle and high performance jets use. They are not "carbon-fiber" either. Carbon pads that are available over the counter are semi-metallic pads that have powdered carbon added to them to improve the high and low temperature properties. Mostly they have the cold friction of a good mild semi-metallic with the high temp properties of medium aggressive full metallic. Even the full race, high metal/carbon pads seem to have a fairly wide effective heat range. They are fairly good on the discs too. Since they work so well over a broad range, carbon pads seem to have taken over the high-performance street pad market. The only drawback that these pads have is cost. They are pretty pricey. They also leave lots of black, corrosive, sticky, brake dust so clean your rims regularly. The full race carbon pads seem to eat discs pretty well too. Carbon pads are a flat dark gray to black with a flinty look. [Top]
Brake Doctor - Reducing Green Fade
The way to eliminate green fade is to properly bed or break in your pads before you have to use them hard. The key is to get rid of the volatile elements of the binder resin without overheating or glazing the pad. Ever seen your brakes smoke? - That smelly stuff is the volatile resins being cooked out of your pads. Bedded pads will not smoke very easily.
It is better to bed new pads in on older discs. Older discs are seasoned and more dimensionally stable making them less likely to warp or crack while bedding. Older discs for some reason are less likely to glaze new pads. You should always run a new rotor in with bedded pads also for the same reasons.
When replacing your pads, you could lightly sand your discs with an electric drill and a 220 grit sanding disc, putting a light cross hatch pattern on them. This helps break the glaze on the disc and aides in bedding the new pads quickly. Install your new pads and go for your bedding run. Before making the first stop after changing pads pump the brake lever carefully before you really need to stop. The pistons are fully retracted into the caliper when you change the pads and the lever will feel long at the first brake application.
When bedding in the pads, be very careful as the brakes will not work at their optimum until fully bedded in...
Harder, high temperature pads usually have an overall lower coefficient of friction even when they are in their ideal operating temperature. Because of this you can expect having to pull on the brake lever much harder with them installed unless you go to a bigger brake system with more pad area. Softer, lower temperature pads generally have more initial bite and require less lever effort but they will fade much more quickly.
Through proper selection of brake pad material and careful bedding in you should be able to reduce pad fade to a manageable level except in the most extreme racing conditions. [Top]
Brake Doctor - Reducing Fluid Fade
Fluid fade is caused by the boiling of the hydraulic fluid in the brake system usually in the calipers and even sometimes the lines under hard use. This localized boiling allows bubbles to form in the brake’s hydraulic system. Since air bubbles are compressible, the end result is a long and mushy brake lever / pedal. In extreme conditions the lever will go all the way to the grip without much deceleration of the bike...
Brake fluid is hydroscopic which means it has an affinity to water and absorbs water from the air. When brake fluid absorbs water it’s boiling point drops rapidly. That why it is important to use only very fresh brake fluid, preferably from a recently opened bottle where the factory seal has just been broken. When bleeding brakes, keep the bottle capped except when you are pouring the fluid out. It is also a good practice to keep the cap of the master cylinder reservoir on, but only loosely screwed about ½ turn while you are bleeding, as the brake fluid pulls in the humidity from the air thus you want to minimize its exposure to the air.
You should also bleed your system and change your fluid at least once a year to get the moisture laden old fluid out. Your brake system will last much longer this way as the moisture in old fluid causes corrosion of the brake systems internal parts. If you are racing the fluid changes should be much more frequent than that.
Fluid fade can be avoided nowadays to a large degree with modern high-performance brake fluid, upgarding to DOT 5.1 from the standard DOT 4 but mostly by frequently changing the brake fluid. [Top]HEL Performance Brake Doctor - Brake Discs
Brake discs are used on the front and rear of most modern motorcycles but some older bikes or those with a lesser performance in terms of top speed have been deemed suitable for only using a drum brake on the rear.
Brake discs have a rotor (the disc) which is clamped on by a friction material called the brake pads. The main advantages of using brake discs instead of drum brakes are:
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More resistant to brake fade
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Better cooling efficiency
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Water and dirt resistant
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Less maintenance
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Greater surface area for a given weight of brake
This greater resistance to fade is the most important advantage over drum brakes and is due to the friction surfaces are directly exposed to cooling air and because drum deflection is eliminated from the system.
The hottest part of the braking system is where the friction material contacts either the drum or disc. For a drum brake to cool down the entire drum has to increase in temperature then the drum is cooled by the air around it. The disc however is cooled immediately by air blowing on the discs rubbing surface.
This makes the contact surfaces 'potentially' more liable to damage from corrosion from dirt or water contamination but it reality the constant wiping action of the pads on the discs keeps the surface clean. Centrifugal force also throws material off the brake disc as the disc rotates.
Disc brake pads are easier to change than drums and they are also easier to adjust. Disc brakes are designed to run with little clearance and are self adjusting when they are applied. Disc brake swept area (the area swept by the pads when brakes are applied) is larger than the swept surface area of the drum as the drum only has one side (inside) swept.
Swept area of a brake is an important measure of it's effectiveness. The swept area of the disc is the total area contacted by all the brake pads in one revolution of the disc. This combined swept area of all brakes can be divided by the weight of the bike to give an indication of how effective the brakes are likely to be.
Some discs have slots or holes machined in them to reduce the hot-gas and dust particle build up between pad and disc. This reduction reduces pad glazing and when considered in terms of racing when pads are larger and temperatures are high hot gases have a harder time trying to escape than with small pads.
Discs are also available with a 'wavey' designed outer ring which offer strong initial grip, perfect pad cleaning, greater perimetal surface with greater heat dissipation, possible expansion towards the middle circumference avoiding conic deformations and less weight.
Recently a new design of rim mounted brake discs have also been available to the public and can be seen on oem motorcycles such as the new Buells. These discs offer some more excellent characteristics such as a much larger surface area offering greater cooling efficiency, larger swept area offering a much longer wearing and more powerful brake disc, zero torsional load, and significantly reduces unsprung weight.
HEL Performance Brake Doctor - Brake Drums
Most modern motorcycles use disc brakes, but some still use a drum brake on the rear. Even though drum brakes share common features, details may differ.
Each has a metal drum, usually cast iron, which rotates with the wheel. Within the drum are brake shoes, lined with friction material. The brake shoes are moved against the inside of the drum by pistons inside the wheel cylinders. Hydraulic fluid under pressure in the wheel cylinders moves the pistons. Wheel cylinders and brake shoes are mounted onto a metal backing plate which is bolted to the axle or the suspension upright.
The brake drum is a large critical part of the braking system. If the drum is too small or flexible the brake will perform poorly under severe use no matter how good the system may be. The important properties of a good drum brake are:
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Must have a hard wear-resistant rubbing surface and the surface finish must not damage the lining.
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Must be strong enough to withstand the hardest braking at high temperatures.
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Must be stiff and resistant to distortion and warping.
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Must dissipate heat rapidly and withstand excessive temperatures.
HEL Performance Brake Doctor - Master Cylinders
Brake fluid movement and pressure are created by the master cylinder. The difference in piston size means that by the time the braking force reaches the calipers it has been increased to about 26 times your initial input at the lever...
Brake Doctor - Single Master Cylinder
The simplest master cylinders have a single piston to operate both front discs or the single rear disc on a motorcycle. In cars they have been replaced by a tandem master cylinder in which there are two pistons - one operating front right hand and rear left hand brakes and the other operating the front left and rear right hand brake so that in the event of failure two opposite brakes still work.
Obviously this isn't able to be replicated on a motorcycle braking system with only two discs instead of four but AP Lockheed have copyrighted the idea to manufacture a twin master cylinder. One piston operates the right hand disc and the other the left hand disc which is a good safety device in that with the failure of one caliper or hose the other can still operate meaning some braking ability is retained in the system.
Most hydraulic fluid reservoirs are integral with the master cylinders and are directly above the piston but some have a remote reservoir connected to the master cylinder via a small hose (Brembo for example). the piston is moved forward in the cylinder via the brake lever. The piston is returned by a spring against the piston when lever force is removed. There are two holes, called ports, in the master cylinder which allow fluid to enter the cylinder. These are called the fluid inlet port and the compensating port. When the piston is at rest the compensating port is just ahead and the fluid inlet port is just behind the lip forming the front face of the piston. In this position the piston rests against a clip or retaining ring that prevents the spring return from pushing the piston out of the end of the cylinder.
As the brakes are applied the piston moves forward with the lip of the seal covering the compensating port. Before this port is covered, piston motion forces excess fluid back into the reservoir. If you look into a reservoir just as the piston begins to move you will see slight fluid motion. Once the compensating port is covered, fluid trapped in the hydraulic system cannot escape provided there are no leaks in the system. Beyond this position the piston moves fluid to apply the brakes.
Initial fluid movement reduces clearances in the system. When all clearances are eliminated the fluid stops moving and pressure rises to the maximum you and the system can obtain. When the lever is released hydraulic pressure is reduced, the disc pistons retract and the master cylinder piston moves back to the retaining pin due to the spring return operating.
The fluid inlet port function is more complicated. The piston has small holes drilled into it through the front face lip. the cup seal is installed ahead of this face with the lip extending forward. the fluid inlet port allows fluid behind the front lip of the piston, the holes allowing fluid to contact the seal face. this helps with brake release when the piston returns. As the brakes are released the return spring may move the piston faster than the fluid can move. When this occurs the seal lip is drawn away from the cylinder wall allowing fluid to flow through the small holes in the seal lip. This keeps fluid ahead of the piston at all times and prevents the formation of tiny bubbles in front of the piston which would cause a spongey brake with excessive travel. The secondary seal stops any fluid leaking out of the open end of the cylinder.
HEL Performance Brake Doctor - Bleeding The Braking System
Experienced bike owners and mechanics will tell you that brake bleeding is easy. It is, but there is plenty of potential for error. Reading this guide will not turn you into an expert overnight. We have made every attempt to be accurate and easy to read but we cannot impart the gifts of skill, experience and common sense. If after reading this page you feel inclined to carry out bleeding to the braking system of a bike we will not accept responsibility for what happens next. You are responsible for your own actions and this page has been placed online to only offer an introduction into bleeding. We will obviously be happy to answer any questions you may have via brakedoctor.
Brake Doctor - What you will need...
Even though it is possible to bleed bike brake systems on your own it's advisable and much easier if two people do it. You will need clean, fresh brake fluid which has been allowed to settle over night - do not shake the bottle before starting as this will put air bubbles into the fluid, a length of plastic tubing which fits tightly onto the bleed nipple and a glass container so you can see the air being expelled from the system.
It's a good idea to cover any areas around the master cylinder and the bleed nipples to protect from accidental spillages. The area around the master cylinder and the bleed nipples should be as clean as you can get them as there is no point getting dirt into the system.
Firstly you need to remove the old lines so attach the plastic tubing to one of the bleed nipples and open slightly so you can pump most of the old fluid out before you take the old hoses off. It's not unknown for the bleed nipples to be seized in the calipers - mild steel nipples and alloy calipers will suffer electrolytic corrosion naturally and winter salt in the UK will only increase the effect. You may want to take the opportunity to replace the mild steel nipples with stainless steel ones.
Assuming that you have been able to undo the bleed nipples make sure the brake reservoir has plenty of fluid in it and then rest the cap back on top to stop fluid squirting out when you begin bleeding.
Brake Doctor - Priming (Filling) The System
If you have a twin disc system bleed one caliper at a time. Attach the tube to the bleed nipple and place the other end in the clean glass jar. Poor some clean brake fluid into the jar so the end of the tube is submerged so you don't pull air back in to the system.
Then open the bleed nipple, squeeze and release the brake lever slowly to give the master cylinder enough time to suck in fresh fluid from the reservoir.
Keep an eye on the master cylinder reservoir and make sure the fluid level does not fall below the minimum mark else you will start sucking air into the system. Fluid may be being pulled into the system from the jar and you may see the level drop - this is fine but again make sure the end of the tube is always immersed in fluid. It shouldn't take too many lever actions to fill the system. Tighten the bleed nipple when finished.
Brake Doctor - Bleeding The System
Open the bleed nipple slowly - you should only need half a turn and at the same time slowly and smoothly squeeze the brake lever in (or push the brake pedal down). Hold the lever in and you should see air bubbles or fluid being expelled into the jar. Old brake fluid looks foul and can be any colour from dirty white to brown or black. Movement of fluid and/or bubbles will continue for a second or two, close the nipple and then release the brake lever. Check the fluid level in the reservoir and top up if necessary. Repeat this operation until no more bubbles appear and the fluid coming out is clear.
If you have a twin system repeat this process with the other caliper (it's best to do the furthest away from the master cylinder first) If everything has gone okay you should now have a brake system with a good solid feel to it, the lever will travel a short distance and then a solid resistance will stop it moving any further.
If when you continue to apply pressure you get a slow movement or spongy feel to the lever it's a good sign that there is still air in the system. There are a number of possibilities not least that you didn't get all of the air out of the system so you should start bleeding again. Tighten all parts to the correct torque setting and then check the whole system to see that the lines are not trapped on full lock, no fluid leaks from anywhere etc.
Brake Doctor - Troubleshooting
Not all calipers have there bleed nipples at the highest point on their anatomy. This means that if there is a small pocket of air trapped above the nipple it will be hard to remove (air always goes to the highest point of the area it is in) and make the system spongy. You can get around this by taking the caliper off and making sure the nipple is at the highest point but remember to put a spacer in between the pads to stop the pistons popping out and making it easier to refit the caliper.
A similar problem occurs with some racing bikes which have steeply angled handle bars - the brake hose arches up above the master cylinder and a small pocket of air can get trapped here. Again you can rearrange the layout or you could inject brake fluid using a syringe very carefully and slowly in through the bleed nipple in the caliper bearing in mind that the fluid in the reservoir may overflow. Fitting a banjo bolt which incorporates a bleed nipple to the master cylinder and bleeding this first before the rest of the system is another way to fix this problem.
If you are unable to remove the sponginess no matter how carefully you bleed the system you may have a sealing problem which you will need to consult your local dealer about.
The master cylinder is fed from the reservoir by a tiny hole and this hole easily gets blocked which is why cleanliness is so important when bleeding brakes. If you cannot bleed your brakes yourself make sure you talk to your local dealer and get them to do it for you.
Don't be tempted to use any of the 'self bleed' gadgets unless you really, positively, nothing else for it, have to. These things allow the nipple to be kept open as they incorporate a non return valve to stop air re-entering the system. But the bleed nipple has a threaded end which screws into the caliper - air can be sucked into the caliper along this route if the nipple is loose in the threaded part of the caliper - it will only be a small amount but why do it in the first place as we are trying to remove air...
When you have successfully bled your brakes make sure both bleed nipples are done up tightly, all banjo bolts are done up tightly and top up the reservoir. Do not overfill the reservoir as this can cause hydraulic locking of the system preventing the pistons in the caliper from fully retracting - this causes binding of the brake.
Re check the system visually before test riding - and we mean test riding. Just go forward a few feet slowly and apply the brakes then bring the bike back into the workshop to check that their is no fluid leaking from the system, everything is done up correctly and the brakes have a good solid feel to them. Do not ride your bike until you are certain you have bled the brakes correctly - If in doubt get your local dealer to bleed the system for you.
Brake Doctor - Troubleshooting And Trade Secrets!!!
Bleeding brakes like many things is a skill and with some modern master cylinders being mass produced sometimes rough areas within them can catch and air bubble which you simply can't remove with normal bleeding procedures. In these cases you can use our handy 'trade secrets' which may well help.
1 Tying the lever back overnight
One way to rid any last remaining air from the system is to tie back the lever over night. Firstly remove the master cylinder reservoir cover BUT then balance it back on top of where it should be fitted so that moisture in the air does not contaminate the system overnight. Then pull the lever in as hard as possible and cable tie it in the ON position (brake lever pulled right back to the bars). Leave this overnight and in the morning release the lever and your brakes should feel fantastic - basically the back pressure caused by tying the lever back overnight will have forced any last remaining air out of the system out to the atmousphere and leave you with sharp brakes the next day. Replace the reservoir cap correctly and you are ready to ride.
2 Priming the system the easy way
You will have read above the normal way of priming the system but another way (used by many professionals) is to connect the hoses to the master cylinder as per normal but instead of fitting them to the calipers you simply put the caliper end or ends of the brake lines INTO the reservoir at the top so that the banjo holes or fitting hole is beneath the surface of the brake fluid. Make sure you have towels etc covering all areas around the master cylinder in case of spillage etc. SLOWLY pump the system in and out (not fast as brake fluid will squirt out over paintwork etc) as this 'circuit' will completely fill the hoses with fluid making bleeding much easier afterwards. On the final 'squeeze' of the lever leave it on (fully pulled in) and tie it back. Remove the hoses from the reservoir and wipe off all excess fluid. Then connect them to the calipers at the correct torque and then release the lever. Bleed as normal but with the added benefit of the lines and master cylinder being air free to start with!!!
3 Reverse Bleeding
Although we aren't fans of this type of bleeding I do include it here as many bike shops use this themselves - the only thing I would say is if you push the fluid in too quickly you can pop brake caliper seals etc so slowly and low pressure are the key words here!!!
For this you will need a large syringe filled with new fresh brake fluid and a tube which fits the bleed nipple and the syringe. Connect the syringe tube to the bleed nipple and open the bleed nipple - SLOWLY push the brake fluid in through the caliper up to the master cylinder (which is bleeding in reverse) Do the caliper furthest from the master cylinder first and then the other one. Repeat if necessary and keep checking the reservoir level so you do not overfill the system. This way ALL AIR IS PUSHED UP AND OUT OF THE SYSTEM meaning bleeding is much easier than trying to push air DOWNWARDS and out of the system in conventional bleeding BUT please do not exert massive pressure at the caliper as this can pop seals etc.
Ride Safe.
HEL Performance Brake Doctor - Frequently Asked Questions
Can you use any fluid in the braking system?
What types of brake fluid are available?
What are the boiling points of brake fluids?
What are the pros and cons of fluids?
Do -02 lines give more pressure than -03 lines?
What is the difference in performance between two full length front lines and an over the mudguard front kit?
What's better - three front lines using the oem splitter, two full length fronts or an over the mudguard front kit?
Brake Doctor - Can you use any fluid in the braking system?
No you cannot. The fluid in the braking system must be a designated brake fluid which meets with certain specifications - DOT3, DOT4, DOT5 or DOT5.1 are the most common, and your owners manual will inform you of the fluid used as standard in the system. Don't use aircraft hydraulic fluid (even if you can get hold of it) - it isn't the brake fluid equivalent of Avgas, and do not use the mineral based fluid formulated for Citroens either.
Brake Doctor - What types of brake fluid are available?
There are basically two types of fluid: glycol-based and silicone-based. You can use one or the other, but NEVER mix the two fluids. If you want to use a different fluid than the one in your system (glycol to silicone / silicone to glycol) then you will need to flush the system before changing. Brake fluid is available in different specifications to meet American Department Of Transport (DOT) requirements. These DOT regulations are occasionally updated but basically the higher the DOT rating the higher quality, higher boiling point and more expensive the fluid. DOT3 is the basic brake fluid. DOT4 is a higher standard and both of these fluids absorb water - they are hydroscopic, but DOT4 fluid contains additives which prevent water affecting the fluids performance to the same degree. DOT5 was originally formulated for silicone based brake fluid although there is now a new DOT5.1 standard for glycol based fluid which has some of the desirable properties of silicone fluids. Both types - glycol based and silicone, have different pros and cons though.
Brake Doctor - What are the boiling points of brake fluids?
Brake fluid works in a harsh environment being close to the heat generated by the braking system the fluid heats up and under extreme conditions such as racing this can lead to the fluid boiling. This manufacturers found that by increasing the boiling point the fluid performed better in use as when fluid boils it leads to cavitation within the system and loss of braking pressure. The boiling points are as follows:
| Minimum Specification | Good Brand Min Spec | |||
| DRY | WET | DRY | WET | |
| DOT 3 | 205°C | 140°C | 220°C | 150°C |
| DOT 4 | 230°C | 155°C | 260°C | 170°C |
| DOT 5 | 260°C | 180°C | 270°C | 190°C |
| DOT 5.1 | 260°C | 180°C | 270°C | 190°C |
Wet figures above show the drop in performance when the fluid has a certain water content (less than 3%). For example when DOT5.1 is contaminated with 3% water (which is considered a lot) performs almost as well as basic DOT3 in prime condition. The most important thing to do is regularly change your fluid - we suggest every three months but manufacturers suggest at least once every twelve months. After six months use a typical DOT3 fluid may have had it's boiling point lowered from 205°C to about 165°C.
Brake Doctor - What are the pros and cons of fluids?
DOT3 and DOT4 are hydroscopic - they allow water to be absorbed into the fluid. This means that when heated up a fluid with water in it will boil more quickly (water boils at a lower temperature than brake fluid) which will lead to cavitation. DOT5 does not absorb water and will not strip paint work but as the water is not absorbed it simply sinks to the bottom of the system which is usually the caliper where operating temperatures are very high. Water in the caliper is bad as it can cause corrosion within the caliper although silicone fluids contain corrosion inhibitors and as water has a lower boiling point than fluid (100°C) there is more chance of boiling under relatively light braking. Glycol based fluids can strip paintwork where as silicone based do not. Silicone fluids also have a very long life which is why it is used in military vehicles which may be stored for years without use but be expected to be ready for action at a moments notice.
Brake Doctor - Do -02 lines give more pressure than -03 lines?
No - the fitting of the smaller dash 2 hoses sold by some manufacturers makes NO difference to the pressure produced at the caliper as is commonly believed. There are no substantial benefits associated with the use of dash 2 hoses except that the manufacturer can charge you more for them. The only way to increase pressure coming out of the system is to increase the pressure going into the system. 500 psi in equals 500 psi out - fluid cannot be compressed to a lesser volume, no matter how high the pressure and pressure is equal over all surfaces of the containing system.
Brake Doctor - What is the difference in performance between two full length front lines and an over the mudguard line kit?
Nothing except for the way the system looks. Both systems operate in exactly the same way and give you the same performance. As the system is a closed hydraulic system without air in it when the fluid moves at one end the same movement takes place at the other end. Because they have different lengths doesn't mean that the system will work slower at one end than the other. Imagine pushing two sticks away from you which are both 6" long - the ends both move at the same time. Now imagine pushing two sticks - one is 6" and the other is 36" - the ends still both move at exactly the same time. So the over the mudguard gives the same performance as the 2 full length - it's just which you prefer.
Brake Doctor - What's better - three front lines using the oem splitter, 2 full length fronts or 2 fronts with one over the mudguard?
As we have said above the three systems give the same performance but the three line front has an additional 2 possible leak points in the system, is heavier and harder to bleed than 2 front lines. It is easier to fit though as you follow the oem setup. Another thing to remember is do you race or use the bike on track days? If so go for the 2 full length front as in the UK one of the ACU rules states that you are not allowed an over the mudguard system unless it is an original system.
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