DESCRIPTION
Hydraulic system bleeding is necessary any time air has been introduced into system. Bleed brakes at all 4 wheels if master cylinder lines have been disconnected or master cylinder has run dry. Bleeding can be accomplished by using pressure bleeding equipment or by manually pumping brake pedal and using a clear bleeder hose.
BENCH BLEEDING MASTER CYLINDER
Note. Bleed tubes must have a residual pressure check valve installed to keep tubes from siphoning brake fluid.
- Clamp master cylinder in vise by mounting flange. Install and tighten threaded end of bleed tubes in outlet ports of master cylinder with opposite end of bleeder tube in reservoirs. Fill reservoirs with clean brake fluid so that bleed tube ends are submerged in brake fluid.
- Slowly compress and release piston assemblies until bubbles cease to appear in brake fluid. Remove tubes and plug master cylinder outlets to keep fluid from draining.
MANUAL BLEEDING
Note. Ensure that fluid level in master cylinder is adequate at all times during bleeding procedure.
- Fill master cylinder with clean brake fluid. Install bleeder hose to wheel assembly being serviced. Submerge other end of hose in clean glass jar partially filled with clean brake fluid.
- Depress brake pedal slowly through its full travel and hold. Open bleed screw 3/4-1 turn. Close bleed screw. Release brake pedal. Repeat procedure until brake fluid shows no signs of air bubbles.
PRESSURE BLEEDING
Note. On vehicles equipped with power brakes, exhaust vacuum reserve from power unit by depressing brake pedal several times.
- To prevent dirt from falling into reservoir, clean master cylinder and cover/diaphragm assembly. With pressure tank at least 1/3 full, connect to master cylinder using adapters.
- Install bleeder hose to wheel assembly being serviced. Submerge other end of hose in clean glass jar partially filled with clean brake fluid.
- Open release valve on pressure bleeder. Open bleed screw 3/4-1 turn. Close bleed screw when brake fluid is clear and free of bubbles. Bleed remaining wheel assemblies in sequence and in same manner. Remove pressure bleeding tool.
BLEEDING SEQUENCE
Bleed master cylinder before bleeding wheel assemblies. Generally, system is bled starting with wheel assembly furthest from master cylinder and working to wheel assembly closest to master cylinder.
On vehicles equipped with dual diagonal brake system, bleed right rear first followed by left front, left rear and then right front.
BLEEDING PRESSURES
| Application | Psi (kg/cm 2 ) |
|---|---|
| General Motors | 20-25 (1.40-1.75) |
BLEEDING PRESSURES
SERVICING METERING VALVE
- On disc brake equipped vehicles, except those with dual diagonal brake systems, the metering section of combination valve must be held open before pressure bleeding.
- Hold metering valve open while pressure bleeding front brakes. Loosen front mounting bolt and install pressure bleeding tool on combination valve, valve stem should be fully extended or depressed.
Note. Never reintroduce brake fluid that has been drained from hydraulic brake system or that has been allowed to stand in an open container for an extended period of time. Also, do not use fluid that contains a petroleum base. Petroleum based fluids will cause swelling and distortion of rubber parts in hydraulic system.
