About Brake System Bleeding

BASE BRAKE SYSTEM BLEEDING
Bleeding the hydraulic brake system is necessary if air has entered the system. A few causes for this condition are low fluid level, a hydraulic fluid leak, a hydraulic line is opened, or replacement of a hydraulic system component. Symptoms can be noted by an improper or loss of brake operation, and/or a low or spongy brake pedal.

Flushing the hydraulic brake system is necessary if contaminants are found in the hydraulic system. A few causes for hydraulic system contamination are moisture, age of hydraulic system parts and fluid, or improper fluid used in the system. Symptoms can be noted by an improper or loss of brake operation, swollen and deteriorated cups and other rubber parts, and/or a discoloration of the brake fluid.

The hydraulic fluid is bled or flushed from the system through bleeder valves located on the calipers, wheel cylinders, and some master cylinders. When bleeding the hydraulic brake system, use only specified brake fluid, and never reuse old brake fluid removed from the system.

PRESSURE BLEEDING
Pressure bleeding is recommended for all hydraulic brake systems. It is the fastest method because the master cylinder is automatically fed brake fluid from the pressure bleeder reservoir, and no pedal pumping is needed so only one person is required to perform the procedure. However, if pressure bleeding equipment is not available, the hydraulic system may be bled as described under "Manual Bleed."

When pressure bleeding, to prevent air from getting into the hydraulic system, do not shake the pressure tank. Set the tank in the required location, bring the air hose to the tank, and do not move it during the bleeding operation. The tank should be kept at least one-third full.

The bleeder valve should be opened at least one full turn, and intermittently closed at about four-second intervals. This gives a whirling action to fluid in the hydraulic system, and helps expel the air. Refer to Hydraulic Brake System/Specifications for proper wheel bleeding sequence.

PRECAUTIONS
Normal pressure from the pressure bleeder should not be greater than about 35 PSI. On vehicles equipped with plastic reservoirs, do not exceed 25 PSI bleeding pressure.

On models with hold-off valves contained in the combination valve, the valve stem on the outside of the combination valve must be held in position during bleeding using valve holding tool No. C-4121, or equivalent.