1990 Chevy S-10 Master cylinder

To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced.

Test the booster with engine turned off and pumping the brake pedal until pedal goes hard. Then hold your foot on the brake pedal and start engine. If booster is working properly the pedal will go soft as it should for normal use. However, if booster is not working the pedal will stay hard.

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