PRESSURE WARNING (PDWA)

On models with a split braking system, a Pressure Differential Warning Actuator is incorporated in the system to warn of brake failure in either of the braking circuits. On some models the PDWA is incorporated in the master cylinder, but where a separate unit is used this replaces the three-way brake pipe connector at the right-hand side of the engine compartment. The unit incorporates a shuttle valve piston assembly which, when moved from the central position by a pressure differential in the two braking circuits, presses upwards on the warning switch plunger and thus activates the brake failure warning light. Resetting The brake failure warning light on the instrument panel should be checked periodically by pressing the test-push when the light should glow. If not, this indicates that the warning light bulb is blown. When the brake pedal is pressed hard, the light should go out and stay out when the pedal is released. If the light does not go out the pressure in the system in unbalanced or the PDWA unit or its electrical switch are faulty. The switch can be checked by unscrewing it from the PDWA unit and depressing the switch plunger; the warning light should then illuminate. If after bleeding the braking system, the warning light stays on, it may be necessary to centralise the shuttle valve piston assembly in the PDWA unit by opening one of the bleed valves and gently depressing the brake pedal until the warning light goes out. If this does not work at the first bleed valve, repeat this procedure for one of the bleed valves in the other braking circuit, Overhaul If required, the PDWA unit can be dismantled for inspection after removal by unscrewing the end plug from the unit body . Also unscrew the nylon switch, then withdraw the piston assembly. If the piston and cylinder bore are in good condition, fit two new seals to the piston and reassemble the unit. Use a new copper washer when fitting the end plug.