Combination Metering Valve / Brake Warning Switch


All models with dual master cylinders will have a hydraulic system control valve or brake warning switch mounted on the frame rail behind the master cylinder.

Models with disc brakes have a Brake Warning Switch Valve and Metering Valve combined in a single unit (Fig. 1).

Models with drum brakes only will have a brake warning switch.

Figure 1



The brake warning switch is used to indicate a failure in either front or rear hydraulic system. The metering valve is used to better balance the hydraulic system during certain braking conditions (Fig. 1).

Brake Warning Switch Unit

The brake warning switch used in these combination valves are latching types. If a pressure loss occurs in one side of the dual brake system the piston in the valve will move toward the failed side and latch in that position. This will cause the brake system warning light to come on and stay on until the brake system is repaired. After repairing and bleeding the brake system, applying the brakes with moderate force will hydraulically recenter the piston and automatically turn off the warning light.


Testing Brake Warning Switch Unit

The brake warning light bulb function can be tested by starting the engine. The light will go on when the ignition key is turned to the extreme right START position and will go off as soon as the engine starts.

To test the service brake warning system, raise the car on a hoist and open a wheel cylinder bleeder while a helper depresses the brake pedal and observes the warning light. If the light fails to light, inspect for a burned out bulb, disconnected socket, a broken or disconnected wire at the switch. If the bulb is not burned out and the wire continuity is proven, replace the brake warning switch in the brake line Tee fitting mounted on the frame rail.

Metering Valve Unit Operation

The metering valve holds oif hydraulic pressure to the front disc brakes in the 35 to 135 psi range to allow the rear drum brake shoes to overcome the return springs and begin to contact the drums. This feature helps prevent locking the front brakes on icy surfaces under light braking conditions. The metering valve has no effect on front brake pressure during hard stops.

Checking Metering Valve

  1. A slight "bump" can be felt by the foot as the brake pedal is stroked. This bump will occur after the pedal has been stroked about 1 inch.
  2. A visual check will show that the valve stem extends slightly when the brakes are applied and retracts when the brakes are released.
  3. In case of a metering valve malfunction, remove valve and install a new combination valve assembly.

(Combination Brake Warning Switch / Metering Valve)

Due to operating characteristics of the valve, which causes complete shut-off of the flow of brake fluid between approximately 3 to 135 psi, front brake bleeding procedures should be done as follows:

  1. Gravity Bleed: This method of bleeding is not effected by the metering valve, as fluid pressures are always below 3 psi. Remove master cylinder reservoir cover and gasket, then fill reservoirs with approved brake fluid. Open disc brake bleeder screws, and allow fluid and air to drain until stream of fluid is free of air.
  2. Pedal Bleed: This method of bleeding is not affected by the metering valve, as fluid pressures are in excess of 135 psi. Follow normal procedure of pumping pedal and opening bleeder screws. Do not pump master cylinder dry.
  3. Pressure Bleed. This method of bleeding is influenced by the metering valve. Bleed pressure, which is normally about 35 psi, is high enough to cause the metering valve to close, stopping the flow of fluid to the front brakes. However, the valve (Fig. 2) can be held open manually by using Tool C-4121, to pull the valve stem out.

Under no condition should a rigid clamp, wedge or block be used to depress the valve stem, as this can cause an internal failure in the valve, resulting in complete loss of front brakes.

It should be noted that the pressure release valve stem is in its innermost position when there is no pressure present. No attempt should be made to further depress the valve stem.

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A residual valve is required to maintain light hydraulic pressure in the rear wheel drum brake hydraulic system to keep the wheel cylinder piston cups sealed against the piston bore during "oif brake" periods. Residual valves are located as follows.

On RM300 and 1975 M300 models the valve is located in the master cylinder (rear) outlet supplying the rear brakes.

On RM350, RM400, M400, M500, and M600 models the valve (formally installed in the front outlet of the master cylinder) is located in the rear brake system outlet of the brake warning metering valve assembly (Fig. 1).



  1. Remove rear brake tube from the residual valve assembly (Fig. 1).
  2. Remove residual valve assembly from brake warning metering valve assembly.

Figure 1


  1. Install new residual valve in the brake warning and metering valve assembly and torque to 150 inch pounds.
  2. Connect brake line to residual valve, torque to 150 inch pounds.
  3. Bleed the rear brake system at the rear brake tube connection to the flexible hose (Fig. 2).

Figure 2