1972 Dash Brake Light

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As for the valve itself. The device we commonly call a proportioning valve is a actually a combination valve. There are 2 valves in the housing we all reference to as a proportion valve. It has a safety feature, the shuttle valve, and a proportioning feature. The safety feature isolates the front or rear curcuit if a excessive amount of fluid is going to it as a result of a leak. It can see a bleed procedure as a leak. If you damage a line or hose, that curcuit is isolated by the shuttle valve. When the valve slides to isolate a curcuit, it trips the electric switch close and the light on dash comes on by finishing the light circuit to ground.
The other part of the combination valve is the proportion function. It is the small cylinder shaped section at the rear of the combination valve assembly that the rear circuit line is attached to. This is the actual proportion valve. It slightly reduces pressure going to the rear circuit relative to the pressure going to the front circuit. There is no delay in this process. The reason for using a proportion valve is mostly do to a difference in required pressure to slow the wheel at a given pressure in the brake line. The front curcuit with the disc brake needs more pressure. There is also brake weight transfer and other dynamics in play to set up a brake system properly. This is why there are 2 different combo valves sold. 4 wheel and 2 wheel disc brakes.