GM 91174789 Hydraulic Power Brake Booster Assembly

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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. Power brake booster units should not be disassembled due to their complexity and critical relationship to brake performance; it is best to replace a defective booster unit with a new or rebuilt one. To remove the booster, first remove the brake master cylinder, then locate the pushrod clevis connecting the booster to the brake pedal, remove the cotter pin, and slide out the clevis pin. Disconnect the hose leading from the engine to the booster, being careful not to damage it, and remove the four nuts and washers holding the brake booster to the firewall, which may require a light for visibility. Slide the booster straight out from the firewall until the studs clear the holes. For installation, procedures are basically the reverse of removal; check the length of the booster pushrod before installing and adjust as necessary, ensuring the pushrod locknut is tightened securely and the booster mounting nuts are torqued. If replacing the power booster unit, measure the clearance between the master cylinder piston and the pushrod in the vacuum booster, applying a vacuum to the booster and measuring the distance from the end of the vacuum booster pushrod to the mounting face. Subtract the two measurements to determine clearance, adjusting the pushrod as needed to meet specifications. After final installation of the master cylinder and brake hoses, adjust the brake pedal height and free play, and bleed the system.