[markeb01]

I’ve never heard of a drum/drum system needing a proportioning valve. Any system bias is built into the design by the size of the wheel cylinders. Since modern prop valves are actually referred to as “combination” valves (proportioning and warning switch) I would assume what you are asking about is a circuit failure warning switch. It is a simple ball check valve that moves off center activating a warning light if pressure on both sides is not equal. Even though I have a 1975 prop valve on mine, I’ve never connected a warning light to the switch. If one system fails it should be pretty obvious in the way the truck stops with only two wheels doing all the work.