Adjustable prop. valve ?

[Jonboy]

I am looking for some help selecting an adjustable prop valve for my truck. It is a '74 Crew cab (originally 3/4 ton). I am using the 3/4 ton master cylinder and booster (JB1, IIRC), D52 front calipers (already on the truck, 3/4 ton spec), and a 1/2 ton 12 bolt with drum brakes. I am having problems getting the original prop valve to recenter, so there is no fluid going to the rear brakes. I may swap to rear discs in the future, but probably not at this time. The 3/4 ton master has equal size fluid reservoirs, so I am guessing that the fluid volume front/rear is the same as well. Any suggestions to which prop valve to get (ideally adjustable), or any other problems with what I have?