Bleeding The Brake System - Motorsports Tech Session

Grant Loc
September 10, 2011

Hello, how are you today?

Today I want to explain the correct way to bleed a brake system in a race car.

Once the braking system, brake lines and the pedal box have been installed correctly and professionally it will need the air flushing out of the lines. This is called Bleeding the brakes.

The reason why the air needs to be flushed out and replaced with brake fluid is air compresses and brake fluid does not. The air is what normally creates a spongy pedal.

To bleed the system you will need some pipe, a jar / container and brake fluid. First thing to do is to push the pipe onto the brake nipple, then open the bleed nipple with a spanner. You must make sure that the end of the hose is submerged in brake fluid and in the jar / container. Start with the furthest wheel from the master cylinder, and then progressively work to the closest wheel cylinder / caliper.

Have a friend to press the brake pedal to the floor, the pedal must be pressed its full travel and the master cylinders is fully compressed. Air and fluid will escape in spurts, make sure when the assistant returns the pedal this is done very slowly. Continue this process until all the air has escaped and a solid column of fluid is being pushed out when the pedal is pressed. When a solid column of fluid is observed the pedal needs to be pressed all the way down and then the nipple can be tightened up. Once this has been done move to the next wheel.

Be sure to check the fluid level after each wheel, the level will drop very fast. If you let the fluid run dry you will push air back into the system, this will result in starting the process from the start.

With some disc brake calipers it can be difficult to get all the air out of the system, it is a good idea to use a plastic or wooden mallet to tap the calipers this will assist the air to be dislodged out of the system.

I Still Have a Spongy Pedal?

If you have correctly bled the system and are sure that no air is in the system and you still have a spongy pedal then I have a few suggestions why this has occurred.

1) Leaks in the system, check the total system for leaks.

2) Worn out wheel cylinders seals, caliper seals or the master cylinder seals.

3) Insufficient fluid in the master cylinder, check the reservoir fluid level.

4) The brake fluid is boiling at some point in the system that will produce air bubbles.

Thank you for your time, I hope this has been of assistance to you.

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Grant Loc has been involved with Motorsports for over 15 years and the Director of obp Ltd. obp Ltd is a major player in Manufacture and Supply of Quality Race Car Products and provides most of the leading Motorsport distributors all over the World. obp manufacture Race Pedal Boxes, Hyd Handbrakes, Alloy Fuel Swirl Pots, Alloy Fuel Tanks, Alloy Dry Oil Tanks, Alloy Header Tanks, Alloy Oil Catch Tanks, Seat Brackets etc. http://www.obpltd.com