If this is factory original set up the Prop valve is the Combination valve that does the following: Distribution valve which is primarily a block to distribute fluid to the lines with a moveable piston to identify when you have a pressure difference rear to front and trigger your brake warning light. Additionally you have the Meter Valve which holds off pressure to the front disc until sufficient pressure on rear. This valve is separate on the 3 piece set up and combined with the 2 piece. But since you said 72 it is all in one valve. It also has the proportioning valve to regulate rear brake pressure less then front after a set value.

Since only one front disc affected, I would say a block in that line not allowing pressure to bleed off, or despite the rebuild the piston is sticking. Get a new caliper. The 70-72 stuff is inexpensive at the local auto store. Or put a pressure gauge on the line to see if pressure is not releasing. Can compare good to bad side.