With the ignition off, pump the brake pedal to deplete the booster reserve. If the booster is working properly, the pedal will drop slightly. To test booster function once the reserve is depleted, hold moderate pressure on the brake pedal and start the engine. Check engine vacuum at idle with a vacuum gauge. Inspect the vacuum supply hose to the booster for kinks, cracks or other damage. A pedal that feels hard immediately, or after only one brake application, may have a vacuum leak or a low level of engine vacuum. At least two brake applications should have a power-assisted feel before the pedal hardens noticeably. With the engine off, apply the brake pedal repeatedly with medium pressure until the booster reserve is depleted. Vacuum Boosters Require Three Basic Tests:Ĭheck pedal feel and vacuum booster function while test-driving the vehicle. As the push rod continues forward, it opens the vent valve to atmospheric pressure, which pressurizes the boost chamber(s) to create a force on the diaphragm(s), power piston and the push rod connected to the master cylinder pistons. In apply mode, the pressure from the brake pedal causes the push rod to move the treadle valve forward and close the vacuum port to the vacuum diaphragm chambers and isolate the vent valve. Vacuum brake boosters have four modes of operation: rest, apply, hold (or balance) and release. Most late-model vehicles use a vacuum booster to help apply the brakes, usually located behind the master cylinder on the firewall. Diagnose hydroboost systems for leaks and proper operation repair or replace parts as necessary refill and bleed system following manufacturers’ specifications.Diagnose vacuum-type power booster units for vacuum leaks and proper operation inspect the check valve for proper operation repair, adjust or replace parts as necessary. Check vacuum supply (manifold or auxiliary pump) to vacuum-type power booster.
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