GM 94850171 Valve,Brake Master Cylinder Propn

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Disconnect the wire from the pressure differential switch by squeezing the side lock releases and pulling up; pliers may assist if needed. Use a jumper wire to connect the switch wire to a good ground, such as the engine block, then turn the ignition key to the On position to check if the warning light in the instrument panel lights up. If it does not, the bulb may be burned out or the electrical circuit could be defective, requiring replacement of the bulb or repair of the circuit. Once the warning light functions correctly, turn off the ignition switch, disconnect the jumper wire, and reconnect the wire to the switch terminal. Ensure the master cylinder reservoirs are full, attach a bleeder hose to one of the rear wheel bleeder valves, and immerse the other end in a container with clean brake fluid. Turn the ignition switch On, then open the bleeder valve while a helper applies moderate pressure to the brake pedal, which should cause the brake warning light to illuminate. Close the bleeder valve before the helper releases the brake pedal, then reapply the brake pedal with moderate to heavy pressure to turn off the warning light. Repeat this process with one of the front brake bleeder valves, ensuring the warning light reacts similarly. If the warning light did not illuminate during the previous steps but does light when a jumper is connected to ground, the warning light switch portion of the combination valve is defective, necessitating replacement of the combination valve since its components are not individually serviceable. To replace the valve, place a container underneath and protect painted surfaces with newspapers or rags, disconnect the hydraulic lines at the combination valve and plug them to prevent fluid loss and contamination, disconnect the electrical connector from the pressure differential switch, remove the bolt holding the valve to the mounting bracket, and take the valve out of the vehicle. Installation follows the reverse order of removal, and the entire brake system must be bled afterward.