GM Prizm/Nova 1985-1993 Repair Guide

Brake System Valves



See Figures 1 and 2

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Fig. Fig. 1: Proportioning valve and master cylinder assembly

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Fig. Fig. 2: Proportioning valve location on the center of the engine bulkhead

The proportioning valve is located on the center of the firewall under the hood on all Prizms without ABS or attached to the hydraulic modulator on vehicles with ABS. Except for leakage or impact damage, it rarely needs replacement. If it must be removed, all 5 brake lines must be labeled and removed and the valve removed from its mount. Clean the fittings before removal to prevent dirt from entering the ports. After the lines are reconnected and carefully tightened to 11 ft. lbs., the entire brake system must be bled.

On vehicles equipped ABS, refer to Hydraulic Modulator, REMOVAL & INSTALLATION procedure outlined in Anti-Lock Brakes at the end of this Section.


See Figure 3

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Fig. Fig. 3: Inspection of the vacuum check valve

The brake vacuum check valve allows vacuum to flow out of the brake booster but will not allow back-flow. This maintains a supply of vacuum within the booster during periods of low engine vacuum. The valve can be removed from the hose by hand. Once removed, the valve can be tested by gently blowing through it. It should allow airflow in 1 direction but not the other. When installing a new valve, make sure it is positioned so that the air can flow from the booster to the engine. Most replacement valves have an arrow showing the direction of airflow.