66 valiant disk brake conversion??

You can find cut-away photos/diagrams of the "combination valve", which became standard for front disk cars by the 1970's. It consists of: proportioning valve, pressure imbalance switch, and front metering valve. Your "hold off" valve sounds like the later. Its purpose is to hold-off pressure to the front calipers until the rear shoes have had time to move into contact with the drum. There are aftermarket version w/ adjustable proportioning (always good), and ones I have seen have the imbalance switch. You can wire the later to short to gnd, with the other wire to your dash e-brake lamp (parallel). Can similarly wire a low-reservoir switch to short to gnd.