When any part of the hydraulic system has been disconnected for repair or replacement, air may get into the lines and cause spongy pedal action (because air can be compressed and brake fluid cannot). To correct this condition, it is necessary to bleed the hydraulic system after it has been properly connected to be sure that all air is expelled from the brake cylinders and lines.
If air has been introduced at the start of the system (the master cylinder reservoir is allowed to approach empty and introduce air or the cylinder fittings are disconnected), then the system must be thoroughly purged of air beginning with the master cylinder. Once the master cylinder has been thoroughly bled, you should bleed one brake cylinder at a time, beginning at the cylinder with the longest hydraulic line (farthest from the master cylinder). Keep the master cylinder reservoir filled with brake fluid during bleeding operation.
Of course, if no air was introduced early in the system, only the cylinder(s) AFTER the point where the lines were disconnected must be bled. If you are only servicing a single caliper or wheel cylinder, that should be the only bleeding point necessary.
It may be necessary to centralize the proportioning valve after a brake system failure has been corrected and the hydraulic system has been bled.
The primary and secondary hydraulic brake systems are individual systems and are bled separately. During the entire bleeding operation, do not allow the reservoir to run dry. Keep the master cylinder reservoirs filled with brake fluid.
See Figure 1
- Fill the master cylinder reservoirs.
- Place absorbent rags under the fluid lines at the master cylinder.
- Have an assistant depress and hold the brake pedal.
- With the pedal held down, slowly crack open the hydraulic line fitting, allowing the air to escape. Close the fitting and have the pedal released.
- Repeat Steps 3 and 4 for each fitting until all the air is released.
WHEEL CYLINDERS & CALIPERS
See Figure 2
- Clean all dirt from around the master cylinder fill cap, remove the cap and fill the master cylinder with brake fluid until the level is within 1 / 4 in (6.3mm) of the top of the edge of the reservoir.
- Clean off the bleeder screws at the wheel cylinders and calipers.
- Attach a length of rubber hose over the nozzle of the bleeder screw at the wheel to be done first. Place the other end of the hose in a plastic or glass jar, submerged in brake fluid.
- Open the bleed screw valve 1 / 2 - 3 / 4 turn.
- Have an assistant slowly depress the brake pedal. Close the bleeder screw valve and tell your assistant to allow the brake pedal to return slowly. Continue this pumping action to force any air out of the system. When bubbles cease to appear at the end of the bleeder hose, close the bleed valve and remove the hose.
- Check the master cylinder fluid level and add fluid accordingly. Do this after bleeding each wheel.
- Repeat the bleeding operation at the remaining 3 wheels, ending with the one closest to the master cylinder. Fill the master cylinder reservoir.