To perform an operating check, depress the brake pedal several times with the engine off to ensure there is no change in the pedal reserve distance, then start the engine; if the pedal goes down slightly, operation is normal. 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 goes down farther the first time but gradually rises after the second or third depression, the booster is airtight. While the engine is running, depress the brake pedal and stop the engine with the pedal depressed; if there is no change in the pedal reserve travel after holding the pedal for 30 seconds, the booster is airtight. Before replacing the power booster unit, inspect the condition of the rubber grommet connecting the check valve or vacuum sensor to the power booster, as a cracked or damaged grommet can cause vacuum leaks. Disassembly of the power unit requires special tools and is not typically performed by the home mechanic; if a problem develops, it is recommended to install a new or factory rebuilt unit. Set the parking brake with the vehicle parked on a level surface, then with the ignition switch off, pump the brake pedal several times to deplete any remaining vacuum in the booster. Disconnect the cable from the negative terminal of the battery, and for 2010 and later models, remove the battery and battery tray. Remove the engine cover from the top of the engine and the knee bolster panel from under the steering column. Without disconnecting hoses or draining coolant, remove the expansion tank and position it to the side. Disconnect the electrical connector from the Brake Pedal Position sensor located on the brake pedal bracket. Squeeze or pry the tangs of the pushrod retainer, then pull the brake pedal to the rear to release the pushrod retainer from the brake pedal arm, discarding the retainer and using a new one on reassembly. Disconnect the power booster check valve and hose, positioning them to the side, and if equipped with a power booster vacuum sensor, disconnect the electrical connector from the sensor and remove it from the power booster. Disconnect the electrical connector from the brake fluid level sensor located on the brake fluid reservoir. Locate the Brake Pressure Modulator Valve assembly below and to the side of the master cylinder, and without disconnecting any brake lines, remove the mounting fasteners and detach the assembly from its bracket to position it slightly forward and downward. Remove the master cylinder, unscrew the two power booster mounting bolts from the front side of the booster, and remove the booster, noting that the bolts are retained in their holes. For installation, follow the reverse of removal, tightening fasteners to the specified torque without over-tightening the power booster mounting bolts, which may impair brake pedal movement. Inspect and, if necessary, replace the gasket between the power booster and the firewall, ensuring the O-ring is properly positioned on the master cylinder sleeve. Always install a new power booster pushrod retainer and ensure it is fully engaged in the brake pedal arm. Bleed and install the master cylinder, then bleed the brake system. Start the engine and depress the brake pedal several times to check if the Check Engine or Service Engine Soon light is illuminated; if so, have the vehicle towed to a qualified repair shop equipped with the proper scan tool to calibrate the brake pedal position sensor. Finally, test the operation of the brakes before placing the vehicle in normal service.
Posted by GMPartsGiant Specialist 
