I mentioned the RPV so he would be aware it's needed. The combination valve he's looking at should be good. As a side note: you can pull the RPV from the front-brake side of a drum/drum master when adding disc up front(pull the brass fitting, remove the spring and rubber check valve, reinsert the brass fitting). You can stay with the original distribution block. Only problem is, you have to monitor the small reservoir as the front pads wear. Bottom line, pressure for the rear drums need to be taken into account. I am running the prop valve I posted and didn't even look to see what was in the master, it was off a 1986 CUCV disc/drum so I left it be and ran it. System is good, but I used everything for a factory setup for a one ton truck... except I'm using that after market combination valve. I would try to setup without an adjustable at first. I think you should try to get close on the first pass.
So the big question: does the combination valve hold the 10psi for the rear drum - OR - does it only manage pressure to the rear during hard braking.
Just know this going into the upgrade, then you can better diagnose your pedal behavior.
Here's what you get in the combination valve: Metering to hold off pressure on the front allowing the rear to apply first. Differential valve for safety, it's what the wire is hooked to for the idiot light. Proportioning so the rear doesn't skid during hard braking(and only alters pressure to the rear during hard braking).
Match components that are designed to work together... give yourself a brake! (pun pun funny.. right?)
j