Bleeding Brakes
Words and Photos: Mark Yanochko
A brake job is more than just replacing the pads and shoes. The base brake system must be checked for trapped air and bled to remove all traces of air from the brake lines. This is especially true if a wheel cylinder or caliper has been replaced.
If air is trapped in the system, the brake pedal feel will be spongy and soft. Air compresses under pressure, brake fluid does not. If any air is in the brake lines when pressure is applied, the air will compress, resulting in a loss of braking effectiveness. This is a safety issue for your customer.
Bleeding the brakes is relatively straightforward. There are three methods—manual, pressure and vacuum. We’ll examine the manual and vacuum methods, with photos, in this How To article. The pressure method, which requires special equipment, will not be covered, as the other two methods are more common.
MANUAL BRAKE BLEEDING
Manually bleeding a brake system requires the help of an assistant, a length of clear plastic tubing, brake fluid and a glass jar. The following sequence should be followed:
• Left rear wheel • Right front wheel • Right rear wheel • Left front wheel
Note: When bleeding the brake system, some air might be trapped far upstream, as much as 10 ft. from the bleeder screw; therefore, it is essential to have a fast flow of a large volume of brake fluid to ensure that all air is purged from the system.
Have your assistant place the plastic hose, A, on the bleed screw of the left rear wheel, B (Figure 1). Place the end of the hose, C, in a glass jar that contains brake fluid, D (Figure 2). Pump the brake pedal, E, (Figure 3) three or four times, then hold it down. Open the bleeder screw at least one full turn. Remember, just cracking the bleeder screw often restricts fluid flow. The brake pedal should drop to the floor. Close the bleeder screw before the brake pedal is released.
Observe the flow of the fluid through the hose. There should be no evidence of air bubbles in the fluid. Repeat this procedure three or four times. Be sure to monitor the level of brake fluid in the master cylinder—there are Max, F, and Min, G, marks on the reservoir, H (Figure 4). Add fluid, as required. Check the pedal travel. If pedal travel is excessive, or has not improved, a sufficient amount of fluid has not passed through the system to expel the trapped air. Continue the bleeding process, as long as necessary, to remove all the trapped air.
After bleeding the left rear wheel, repeat the above procedures for the right front wheel. You will find that the bleeder screw, J, is much more accessible on front disc brakes than on rear drum brakes (Figure 5). Place the plastic hose, K, on the bleeder screw and loosen it to bleed the caliper, L (Figure 6). After bleeding the right front wheel, continue on to the next two wheels.
VACUUM BRAKE BLEEDING
Vacuum brake bleeding is a very simple procedure requiring one person. All you needed is a vacuum pump, M, a fluid reservoir, N, and two pieces of clear, plastic hose. Connect one piece of hose, O, to the pump and the reservoir. Connect the other hose, P, to the reservoir and the bleeder screw (Figure 7). The sequence of bleeding starts at the wheel farthest from the master cylinder and proceeds to the wheel next closest to the reservoir. The last wheel bled will be the wheel closest to the master cylinder.
Attach the plastic hose to the bleeder screw that is the farthest distance from the master cylinder. Apply some vacuum to the system, then loosen the bleeder screw. Watch as the fluid, Q, flows into the reservoir, R (Figure 8). The fluid should contain no air bubbles. Apply more vacuum. When you see a solid stream of bubble-free fluid flowing into the reservoir, re-tighten the bleeder screw. As with the manual bleeding procedure, be sure to monitor the level of brake fluid in the master cylinder (Figure 4). Add fluid, as required.
After you finish bleeding the first wheel, empty the reservoir and bleed the wheel that is next closest to the master cylinder. Repeat the procedure above and continue this process for the remaining two wheels. Always check the fluid level in the master cylinder reservoir before bleeding the next wheel.
Mark Yanochko is a 32-year veteran of the automotive technical, service and repair industry.
