GMC 89040285 Power Brake Booster Assembly

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To check brake booster operation, you should start the engine and step down on the pedal. If the pedal presses a bit, then it is normal. Thereafter, press down on the brake pedal several times with the engine running to determine that there are no changes in its reserve distance. For the purpose of testing for air tightness, you should start your vehicle for one or two minutes and then switch it off. Once this is done, slowly push brake pedal down until either it drops forward first but rises up gradually after second or third depression indicating airtight booster. Additionally, while depressing the pedal with the engine still running stop fueling into it. If there's no change in pedal reserve travel after 30 seconds of holding down that indicates an airtight booster.You need to disable the airbag system and disconnect battery before removing brake booster. Separate vacuum hose and remove master cylinder.Revise left side under-dash panel's removal followed by taking out retaining clip securing booster rod to pedal. Remove four nuts holding brake booster against firewall; slide out till studs clear holes.Slide out until studs clear holes.Remove four nuts holding brake booster against firewall.Pull out until studs clear holes.Remove four nuts holding brake booster against firewall.Take off left side under-dash panel.Pull out until studs go through holes.Pulling out till studs go past holes.Final step: pull gasket along with booster from motor cavity.To install use new gaskets tighten mounting nuts of both master cylinder and boosters.Torque is applied in tightening screws during re-installation.

Start the engine and turn it off after one or two minutes. Slowly depress the brake pedal several times and observe if the pedal goes down farther the first time but gradually rises after the second or third depression, indicating an airtight booster.