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How to Test a Brake Booster Check Valve

by KevinM

Nearly all of today's automobiles are equipped with power-assisted brakes. The power assist system makes use of an ingenious booster that is powered by the vacuum generated in the engine intake manifold. For safety's sake the system must operate for at least one or two brake applications even if the engine stops, and a small check valve is therefore incorporated to maintain the booster vacuum under these conditions. If the booster does not seem to be performing properly it is wise to test the check valve to eliminate it as a possible cause of the problem.

Park the vehicle and open the hood. Locate the vacuum booster. This is a dome-shaped assembly that is mounted to the rear wall of the engine compartment on the driver's side, in-line with the brake pedal. Locate the vacuum hose that runs from the engine intake manifold to the vacuum booster.

Trace the vacuum hose back to the engine intake manifold connection. Undo the hose clamp or clip using pliers or a screwdriver and slip the hose off the connection point.

Blow air into the hose. Use your mouth if you don't mind eating a bit of engine dirt. Alternatively, slip an aspirator bulb over the hose end and squeeze. In a pinch, turkey basters make great aspirators. If the check valve is functioning properly it should prevent you from blowing air into the hose.

Suck air out of the hose. If the line is not blocked and the check valve is functioning properly you should be able to easily suck air out of the hose.

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