How to Bleed Brakes
After performing any brake service that requires opening up a hydraulic connection (caliper repair, replacing a hose, etc.) or if the brake pedal feels spongy or soft, a base brake bleed procedure must be performed.
Words By: Mark Yanochko
Editor’s Note: Brake systems are one of the most critical safety systems on any vehicle. At a minimum, always have your brakes inspected once a year. Because brake pads and shoes wear gradually over time, it’s often hard to detect the subtle loss of brake performance.
After performing any brake service that requires opening up a hydraulic connection (caliper repair, replacing a hose, etc.) or if the brake pedal feels spongy or soft, a base brake bleed procedure must be performed.
Air compresses under pressure, brake fluid does not. If any air is trapped in the brake lines when pressure is applied to the brake pedal, the air will compress, resulting in a loss of braking effectiveness.
Click photos to enlarge.
Photos By: Mark Yanochko
Bleeding the base brake system is relatively straightforward. There are three methods – manual, pressure and vacuum. We’ll examine the manual and vacuum methods, with photos. For pressure bleeding procedures, follow the pressure bleeder manufacturer’s procedure for use of their equipment.
If the vehicle is equipped with an Anti-Lock Brake System, refer to the shop manual for the ABS hydraulic unit bleeding procedures.
MANUAL BRAKE BLEEDING
In order to manually bleed a brake system, you’re going to need the help of an assistant, a length of clear plastic tubing, brake fluid and a glass jar. The following sequence should be followed when bleeding a diagonally split hydraulic brake system:
- Left Rear Wheel
- Right Front Wheel
- Right Rear Wheel
- Left Front Wheel
If the brake system is a front-to-rear hydraulically split system, follow this sequence:
- Right Rear Wheel
- Left Rear Wheel
- Right Front Wheel
- Left Front Wheel
Have your assistant place one end of the plastic hose, A, onto the bleed screw of the appropriate wheel, B (see figure 1). Place the other end of the hose, C, in a glass jar that contains brake fluid, D (see figure 2). Pump the brake pedal, E (see figure 3) three or four times, then hold the brake pedal down. Open the bleed screw at least one full turn (the brake pedal will drop). Close the bleed screw, release the brake pedal only after the bleed screw is closed.
Observe the flow of the fluid through the hose. There should be no evidence of air bubbles in the fluid. Repeat this bleeding procedure until all air is removed from the line. Be sure to monitor the level of brake fluid in the master cylinder reservoir (there are MAX (F) and MIN (G) marks on the reservoir, H (see figure 4). Add clean brake fluid, as required. Continue the manual bleeding process in the proper sequence until all lines are bled properly. You will find that the bleeder screws are more accessible on disc brake calipers than on drum brake wheel cylinders.
VACUUM BRAKE BLEEDING
Vacuum brake bleeding is a very simple process and requires only one person. All that is needed is a vacuum pump, I, a fluid reservoir, J, and two pieces of clear plastic hose – one to connect the pump to the reservoir, K, and the other to connect the reservoir to the bleeder screw, L (see Figure 5). The sequence of bleeding is the same as noted in the Manual Bleeding procedure.
Attach the plastic hose to the appropriate bleeder screw. Apply some vacuum to the system, M (see Figure 6) then open the bleeder screw at least one full turn. Watch as the fluid, N, flow into the reservoir, O (see Figure 7). The fluid should contain no air bubbles. Apply more vacuum. When you see a solid stream of brake fluid, that contains no air bubbles, close the bleed screw.
As with the manual bleeding procedure, be sure to monitor the level of brake fluid in the master cylinder reservoir (see Figure 4). Add clean brake fluid as required.
After you finish bleeding each wheel, empty the reservoir and proceed to the next wheel in the sequence noted in the manual bleeding procedure. After all the wheels have been bled, be sure to top off the master cylinder reservoir.
