Brake Warning Lamp Switches

Fig. 4 Pressure Differential Valve & Brake Warning Lamp Switch:

Fig. 5 Pressure Differential Valve & Brake Warning Lamp Switch:

Fig. 6 Pressure Differential Valve & Brake Warning Lamp Switch:

Fig. 7 Dual Master Cylinder With Built In Warning Lamp Switch:

There are four basic types of brake warning lamp switches as shown, Figs. 4 through 7. When a pressure differential occurs between the front and rear brake systems, the valves will shuttle toward the side with the low pressure.
In the switch shown in Fig. 4, movement of the differential valve forces the switch plunger upward over the tapered shoulder of the valve to close the switch contacts and light the dual brake warning lamp, signaling a brake system failure.
In Fig. 5, the valve assembly consists of two valves in a common bore that are spring loaded toward the centered position. The spring-loaded switch contact plunger rests on top of the valves in the centered position (right view). When a pressure differential occurs between the front and rear brake systems, the valves will shuttle toward the side with the low pressure. The spring-loaded switch plunger is ``triggered'' and the ground circuit for the warning lamp is completed, lighting the lamp (left view).
In Fig. 6, as pressure falls in one system, the other system's normal pressure forces the piston to the inoperative side, contacting the switch terminal, causing the warning lamp on the instrument panel to glow.
Fig. 7 shows the switch mounted directly in the master cylinder assembly. Whenever there is a specified differential pressure, the switch piston will activate the brake failure warning switch and cause the brake warning lamp to glow.

Testing Warning Lamp Operation
The warning lamp should illuminate when the ignition switch is in the start position, and turn off when the switch returns to run. If the brake lamp remains on after the ignition returns to run, check fluid level in master cylinder reservoir and inspect parking brake. If the warning lamp does not turn on during cranking, check for defective bulb or blown fuse.