GM 25878055 Hydraulic Power Brake Booster

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Removal and installation of the power brake booster may trigger a CHECK ENGINE warning light and necessitate calibration of the brake pedal position sensor, which requires a specialized scan tool at a dealer service department or qualified repair facility. The power brake booster is not serviceable; if it fails, it must be replaced with a new or rebuilt unit. To check operation, depress the brake pedal several times with the engine off to ensure no change in pedal reserve distance, then start the engine; a slight drop in the pedal indicates normal operation. For an airtightness check, start the engine and turn it off after one or two minutes, then depress the brake pedal several times slowly; if the pedal drops initially but rises on subsequent depressions, the booster is airtight. With the engine off, turn the ignition key to the OFF position and press the brake pedal several times to remove vacuum from the booster. Set the parking brake, loosen the lug nuts on the left-front wheel, raise the vehicle, and remove the wheel. For ABS-equipped models, disconnect the electrical connector and remove the fasteners for the ABS hydraulic control unit mounting bracket; for non-ABS models, remove the mounting fasteners for the proportioning valve. Label the brake lines to the top ports on the proportioning valve or hydraulic control unit, unscrew the fittings with a flare-nut wrench, and cap or plug all openings to prevent contamination. Remove the master cylinder mounting nuts and electrical connector at the reservoir, then remove it along with the proportioning valve or hydraulic control unit, keeping the brake lines connected. Disconnect the large vacuum hose by removing the check valve from the power brake booster. In the passenger compartment, remove the knee bolster and disconnect the electrical connector for the brake pedal position sensor. Remove the retaining clip and washers securing the booster pushrod to the brake pedal arm, then remove the booster-to-firewall mounting nuts and tilt the booster slightly to detach the pushrod from the brake pedal arm. Carefully guide the booster unit out of the engine compartment, then to install, position the booster and attach the pushrod to the brake pedal arm, install and tighten the mounting nuts, and reconnect the brake pedal position sensor electrical connector. Install the master cylinder with the proportioning valve or ABS hydraulic control unit, reconnect the large vacuum hose, and tighten all fasteners. Thread the brake line fittings into the proportioning valve or ABS hydraulic control unit securely, ensuring not to strip the threads. Finally, bleed the brake system and thoroughly test the operation of the brakes before returning the vehicle to normal service.

The thickness of the outer and inner pads should be measured and compared to specified limits; if any pad thickness is below the limit, all pads must be replaced.