Function Test

With engine off, pump pedal to exhaust vacuum. Hold pressure on pedal when it is as high as it will go. Start engine. Pedal should sink slightly and then hold firmly.
If pedal does not react correctly, test vacuum check valve. Vacuum check valve is located in vacuum supply hose, which connects servo to intake manifold on gas models or to vacuum pump on diesel models.
If air is forced into servo end of hose, good check valve should open and allow air to pass. If air is forced into manifold or pump end of hose, good check valve should seat and not allow air to pass. Replace defective check valve.
Repeat function test for servo. If pedal still does not react properly, check for defects or leaks in vacuum or hydraulic systems. If vacuum and hydraulic systems are good, power assist servo is defective and must be replaced.