Servo & Vacuum

To determine the condition of the diaphragm, remove hose from the servo unit and apply 14 inches of vacuum to the tube opening and hold in for one minute. The vacuum should not leak down more than five inches of vacuum in one minute. If leakage is detected, replace servo. To utilize the engine as a vacuum source, proceed as follows:

Fig 1: Vacuum and Electrical Diagram of General Motors Cruise Master Speed Control System

Courtesy of GENERAL MOTORS COMPANY

1. DISCONNECT SERVO BEAD CHAIN and hose from servo unit, connect engine vacuum directly to the servo fitting.
2. Note position of servo diaphragm. Start engine. The diaphragm should pull in. Clamp off engine vacuum supply line and check for leakage.