Brake System Bleeding

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Thanks once again to Gary Morrell.

About ABS

With the ignition off and the brakes not applied, there is little danger of high pressure in the brake lines. There is, however, fluid under pressure (about 1200 psi) in the ABS accumulator. To be safe, please disconnect the battery while bleeding an ABS equipped vehicle.

Why we bleed brakes

First, brake fluid is hygroscopic, it absorbs water. Water in the brake fluid lowers the fluid's boiling point and rusts the steel parts of the brake system, which are many. Since the fluid is subject to a great deal of heat in the calipers, we'd like the boiling point to be as high as possible. Boiling brake fluid makes for really entertaining (not) stops. Second, if the brake fluid has been boiled in the calipers, say, due to some "spirited" driving, it is useless and needs to be removed.

Selecting brake fluid

Silicone brake fluid is the only fluid that is not hygroscopic, however, the seals in many passenger car brake systems are not compatible with silicone, and, at high temperatures, silicone becomes compressable, making for mushy pedal feel. Because of this, silicone is not a racing brake fluid, contrary to popular opinion. Silicone is commonly used by vintage car restorers to preserve valuable brake systems. I do not recommend it for the street or the track. The fluid that is most at risk for water accumulation in the brake system is the stuff in the plastic reservoir. Not only is it exposed to moisture every time you open the cap, but the plastic reservoir is slightly water permeable, so this fluid is constantly exposed to contamination. There are 2 morals to this point:

1. Before bleeding the brakes, steal the wife's turkey baster and use it to suck the old fluid out of the reservoir. Refill it with fresh fluid and then start bleeding. This way you're not wasting time bleeding old fluid thru the system. Marital bliss hint: buy your own turkey baster.

2. Don't buy brake fluid in plastic bottles: the manufacturer has thoughtfully arranged for it to be water contaminated before you buy it. Buy brake fluid in metal cans, Ford Heavy Duty DOT3 is used by more racing teams then I can remember, and the cans are metal.

General procedure

Don't worry too much about changing the fluid that's in the ABS accumulator. It is isolated enough that there's little danger of water contamination. Start by bleeding the longest line first, that would be the right rear, then move to the driver's rear, then to the right front, and finally the driver's front. Also, on Tauri that have the brake bias adjuster on the rear suspension, make sure that the rear suspension is at normal ride height, if the wheels are hanging, this adjuster reduces the flow to the rear calipers, making the bleeding go really slow.

Specific Techniques

Several methods will be described here - One-man is not my preferred method as there is greater danger that air will be introduced into the system. Read on...

One-man method

For one-man you'll need a container that will stand on its own and a length of tubing that will fit snugly on the bleeder fitting and reach the bottom of the container. Connect the tubing to the bleeder fitting and fill the container with enough fresh fluid to cover the end of the tubing. Now check that the master cylinder reservoir is full. If you didn't empty it and replace with fresh fluid, do it now. Open the bleeder fitting about 1/2 a turn, then climb in and SLOWLY pump and release the brakes about 10 times. Slowly is important here, you're just trying to move fluid, not see how far you can squirt it across the garage. Now check your bleed container and top off the master cylinder reservoir again. DO NOT let the reservoir get empty, it'll introduce a bunch of air into the system that will take you forever to get out. After 20 pumps on each caliper, close the bleeder fitting and make sure you can obtain a firm pedal. Then move to the next caliper.

Two-man method

The bleed hardware is the same but you'll need one man on the pedal and one at the caliper. The caliper man opens the bleeder and says "Down", the pedal guy slowly pushes the pedal to the floor and HOLDS IT THERE, the caliper guy then closes the bleeder and says "Up", the pedal guy then releases the pedal. Repeat 20 times for each caliper, don't forget to top off the reservoir occasionally. The beauty of the two-man method is that there's no danger of pulling air into the caliper around the bleeder fitting threads when the pedal is released. Now you can split a six-pack with your assistant and do some of that male bonding stuff.

Power bleeding

Power bleeding, not to be confused with power lunches or power ties, is where a canister of brake fluid under pressure (about 5 psi), is connected to the master cylinder reservoir and then each bleeder fitting is opened for awhile. The reservations I have with this method are that the fluid is of unknown origin and God knows how long its been in the canister and what its been exposed to. The pressure for the canister is usually provided by the garage's shop air which is normally full of water anyway. Not recommended.

Phoenix Injector

The Phoenix Injector is a "sophisticated hand pump capable of directly pumping liquids and gases (air) at pressures from -20" Hg to 150 PSI." According to the manufacturer "The Phoenix Injector can bleed brakes and clutches quickly and easily-- one man, no compressed air or electricity". See their website for more details.