Vacuum Brake Booster Test Procedure

by H Steve Dolan

If the booster is working correctly, it should have no noticable effect on the engine, but if it is leaking, it will cause big problems. Use the following procedure to check yours.

Things you need:


Hook up the vac guage. Start the engine. Note the reading. This is your "base" reading.

Stop the engine, disconnect the booster vacuum line and cap the port on the manifold. Start the engine, look at the guage. If this reading is substantally higher than the base, you have a leak in the system, probably in the lines leading to the booster.

Stop the engine, remove and test the one-way valve (should be able to blow into it one way but not the other). Air should only pass in the direction indicated by the arrow printed on the top of the valve (make sure to reinstall it in the correct direction).

Reconnect the one way valve and vacuum line to the manifold and start the engine. Watch the vac guage as a friend depresses the brake pedal. It should dip as the pedal is depressed and return to the base reading. If it drops slowly, but returns, you have a blockage/collapse in the vac line. If it falls and does not return, you have a torn diaphram in the booster. Especially indicative of a torn diaphram is a hissing when the pedal is depressed.

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