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Proper Brake Bleeding Techniques
Surprisingly, one of the least understood systems on a race car is the brake system. This is surprising because it is really quite simple. Even a simple system, though, can have some not-so-obvious pitfalls and these catch many racers unaware. Brake issues are one of the most common of the race weekend problems. In this article, we will go through the proper method of bleeding brakes, paying attention to the reasons behind it as well as some critical points where problems often arise.
Step 1. Use proper brake bleeding bottles. These are available from Pegasus, Truechoice and many other suppliers. A bleeding bottle should be transparent so that you can see fluid, fluid color and bubbles. A transparent tube will extend from the bottom of the bottle, through the top and should be long enough to slip over the bleed screw of the caliper while the bottle sits on the ground. Some bottles have magnets attached so they can attach to a rotor or other iron or steel surface. The tube should be flexible enough to slip over the bleed screw and still fit tightly. For very small diameter bleed screws, you can slip a smaller size tubing inside the main tube. Remember that all connections should be tight or air can sneak into the tube, confusing you with bubbles in the hose that did not come from the calipers. The fact that the hose extends to the bottom of the bottle allows fluid to be pulled up into the hose when the pedal is released, rather than air. For that reason, the bottle should have some fluid in it before you begin the bleeding process.
Step 2. Use two bottles, one at each end of the car and on the same side. By bleeding front and rear calipers at the same time, the bias bar at the brake pedal remains relatively perpendicular to the pedal stroke and both master cylinders will travel in a full stroke. If you only bleed one caliper, the master cylinder will not travel through a full stroke because it is limited by the lack of movement of the cylinder at the other end of the bias bar.
Step 3. One person should be in the car. We'll call the pedal pusher George. George should stroke the brake pedal slowly and firmly about three or four strokes. It's not a speed contest, so the frenzied pedal pushing you sometimes see in the paddock is not recommended. Slow and firm is the goal. At the end of the last stroke, George should call out, "pressure" and hold the pedal down. The two bleeders then open their bleed screws about a quarter of a turn. Sometimes this is not possible due to obstructions, and the box end wrench should be slipped off the hex of the bleed screw for a new bite. If the screw is not opened enough, it is not likely that sufficient fluid will be pushed through the hose. With both screws open, fluid will pass out of the caliper, into the hose, carrying any trapped air bubbles with it. You will see them through the hose. When both master cylinders have traveled the full stroke, George will feel the pedal bottom out and stop moving. He should then say, "down". At that time the bleeders should close the bleed screws completely and say, "closed".
In a noisy paddock, it is sometimes difficult to hear these signals. The front bleeder has the responsibility to relay to George the vital information of when the rear bleed screw is closed. Missed timing here allows air to be pulled up into the hose when the pedal is released. The danger is reduced if the bottom of the hose in the bottle is kept submerged. It is also the front bleeder's responsibility to make sure the reservoirs never run low. This will have the same result but the air will be introduced at the master cylinder and a great deal more bleeding will be required to flush it all the way out of the calipers.
Tech Tip - Broken Screws Bleed screws are usually quite small and only a little torque is required to seat them on their taper seats. Bleed screws are often twisted off due to over-tightening, especially those which use wrenches as small as one quarter inch. Use a six point box end wrench to get a good hold on the hex and use a stubby wrench. More screws are broken due to the use of a standard length wrench than any other cause. If a bleed screw breaks, you will have to remove the caliper and use a drill press or vertical milling machine to remove it. Needless to say, this is very inconvenient at the race track. A bit of Copaslip or other anti-seize lubricant on the threads of the screw is also a good idea. Be gentle with the wrench!
Step 4. Step 3 should be repeated until the brake fluid comes out clear, without discoloration and without bubbles. Many times air is introduced at the connection of the plastic tube to the bleed screw. This will give the impression that the caliper still contains air. Make sure the tube fits tightly. When you are satisfied no more air remains in the caliper and lines, close the bleed screws on both calipers and repeat the procedure on the opposite side.
Tech Tip - Brake Flushing If the brake system is empty of fluid, if you suspect the fluid has boiled, or if it has been some time since the system was bled, you should flush fluid through the system. To do this, open both front and rear bleed screws and have George repeatedly stroke the pedal. Ensure the reservoirs always have plenty of fluid and that the tube bottoms stay submerged in the bottle. The stokes should again be slow and deliberate. The goal is to push fluid from the master cylinders through the lines and through the calipers. When it appears most of the air or old fluid has been flushed out, close the bleed screws and pressure bleed beginning with Step 3.
Tech Tip - Bleeding Master Cylinders When replacing a master cylinder, it is sometimes necessary to bleed it. You will know if it is needed by air bubbles coming up through the fluid in the reservoir when the pedal is pushed. Connect a piece of tubing to the output fitting and bend it around so that the fluid coming out is returned to the reservoir. Have George stroke the pedal in the normal slow and deliberate motions. After a few strokes, the bubbles will stop and you will see only clear fluid from the line and no bubbles in the reservoir. The system is then ready to flush fluid through as in the previous tech tip. We carry a piece of dash 3 AN hose with a fitting on only one end for this purpose.
Step 5. When you feel you have removed all air from the system, ask George to try the brake pedal with all bleed screws closed. The pedal should feel firm, travel only a very short distance and then get completely solid. If it seems to 'pump up' i.e. if the first stroke is long and mushy, the next is shorter and firmer and the last is solid, the system still has air in it. Bleed again. If the pedal gets solid but then seems to move down slowly as pressure is maintained, one of the master cylinders is leaking internally, past the seals. It must be removed and replaced or rebuilt. If you see a small spurt of fluid come up through the reservoir when the pedal is pushed, the end seal in the master cylinder is leaking. This also means rebuilding or replacement of the master cylinder.
We use this master cylinder test fixture to check any suspect unit. If it holds pressure for a period of time, we know the seals are working properly. We see higher pressure numbers with larger cylinders.
Step 6. When all the air has been removed from the brake system and the pedal feels solid or even hard, top off the master cylinders with fluid. Screw the caps on firmly, so that they will not vibrate off, but not tight enough that they are difficult to remove. Clean up any spilled fluid. Spray down the master cylinders and surrounding area with brake cleaner and blow dry or at least allow to air dry. Try not to get any brake cleaner into the vent holes in the reservoir tops. Spray brake cleaner on the bleed screws and calipers and blow them dry as well. You should now have a brake pedal that gives the driver confidence.
Tech Tip - Master Cylinder Size On many occasions, we have been asked to troubleshoot an ineffective brake system at a race only to find a problem with the sizes of the master cylinders fitted to the car. I'm going to use a Formula Ford as an example here to make a point. Please understand, though, that other cars with different sized calipers may require different master cylinder sizes than the ones used in this example. The length of stroke of a master cylinder is indirectly proportional to the diameter of the piston in the cylinder. In other words, the smaller the cylinder, the longer the stroke. It's all about fluid displacement. Many times we have discovered a Formula Ford with a 3/4 inch cylinder on the front brakes and a 5/8 operating the rear. Since the calipers on these cars are usually the same size, the pedal travel will be controlled by the front cylinder and the rear calipers will see very little pressure. To make the rear brakes effective with this arrangement, the bias bar will be adjusted all the way toward the rear and this is still sometimes not enough. The result is locking front wheels or brake induced understeer. The cure is to increase the size of the rear cylinder to a .700 inch or even a 3/4 inch cylinder. This will allow the bias bar to be re-adjusted more toward the center and improve the braking function.
On other occasions, we have found the front and rear cylinder sizes switched. With a .700 cylinder operating the front calipers and a 3/4 cylinder on the rears, the bias bar will be adjusted all the way to the front and still the rear wheels will lock or produce a tail happy car under braking. In this case, switch the cylinders. On a dry track with average grip, the optimum brake bias setting will be somewhere near 60% to the front. Other factors like tire condition or size will alter that optimum.
Small master cylinders produce long pedal travel and a generally mushy feel. Large master cylinders give short travel and a hard pedal. Short pedal travel has the advantage of keeping the brake pedal near the level of the throttle allowing more precise heal-and-toeing. Most drivers prefer larger cylinders. To make them effective, the driver's seat must be rigid to avoid the driver being pushed back in it when he applies a lot of pedal pressure. If you run a vintage car with a fiberglass seat, it may need reinforcing to improve your brake performance. In a race car, everything has to work together.
Tech Tip - Caliper Seals The seals in the caliper are square section O-rings. These seals operate in a brutal environment. With temperatures at the rotors which make them glow on a cloudy day, a huge amount of heat is passed through the pads and into the caliper pistons. The seals must stand up to this temperature without leaking. As we all know, heat hardens rubber. This reduces the hysteresis (the stretchiness) of the rubber and will eventually make the seal fail. When this happens, it pulls air into the caliper as well as causing the pistons to stick in the bores. For a low powered car such as a Formula Ford, we recommend rebuilding the calipers once a year in the off season. For high powered cars like our Indy and Formula One cars, the seals are replaced much more frequently. Tracks which are particularly hard on brakes like Road America will warrant caliper rebuilding after each race weekend. It is a good idea to rebuild the master cylinders at the same time as the calipers. Better to do it too often than not enough.
Tech Tip - Brake Fluid Brake fluid is hydroscopic. It absorbs water from the air. It only takes a short time for the fluid in a system to become saturated and as the water content increases, its ability to withstand heat goes down. When brake fluid boils, it turns to gas which is compressible. Since a brake system is designed to hydraulically transmit force, a compressible gas in the system renders it completely ineffective. It is extremely important to keep the fluid as dry as possible as well as to use a fluid which has a very high wet boiling point. The dry boiling point you see frequently advertised is much like the "advertised duration" of a camshaft. It sounds impressive, but has little effect. Brake fluid in the real world - that is once it has been poured into the reservoirs and the brakes bled - absorbs moisture and should be considered wet. When you shop for brake fluid, look for the wet boiling point - not the dry. The chemistry of fluids is sometimes altered to increase the dry boiling point but it comes at the expense of the wet point. As the dry boiling point goes up for advertising purposes, the wet goes down. For this reason, we always suggest fluids with a high wet boiling point regardless of the dry point. Look for what's real. Avoid the smoke and mirrors.
We hope this article has helped you to avoid some of the problems many racers encounter with their brake systems. If you have questions or have something to add, please contact us. We would love to hear from you.
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