power brake booster

To test the booster, pump the pedal 5-10 times when not running, to get rid of all of the residual vacuum. Then, while holding the brake pedal, start the engine and see if the pedal sinks further down when the engine starts; this shows that the booster is properly reacting to engine vacuum.

The check valve should hold some vacuum in the booster. Another way to test if the booster is holding vacuum is to idle the engine, check how far the brake pedal goes down with moderate force, then release the pedal and then shut the engine off off. Then push on the brake pedal with the same force and the pedal should go down to the same approximate position. Push the pedal down a few more times, and it should start not pushing down as far, and get harder to push; this is an indication that the residual vacuum in the booster is being bled off by successive pedal actuations and eventually going away.

If there is no difference in pedal travel with the same moderate pedal force when idling, and then after several pedal actuations when shut off, then the booster is not working, or you have an inadequate vacuum supply.

One thing to to check is that the air opening on the pedal side is clear. This is often a filter felt around the pedal rod area. The can become clogged and block airflow to the pedal side of the booster diaphragm, and acts the same as no vacuum. On some cars, this can be serviced externally, but I am not sure if this is the case on the Mopar units.