Hy 52 Cheese, Okay firstly disc brake master cylinders do not have residual pressure valves built into them, drum brake master cylinders do. Disc brakes do not use or require residual pressure valves, under normal installation circumstances, however in some master cylinder below the level of the caliper installations a low p.s.i. resisdual pressure valve is recommended. If a proportioning valve is not used in a disc/drum system, then rear brake lockup on hard braking is commonly experienced. Builder may have a good thought about something restricting flow to the front brakes, so check your lines up to where they tee for kinks, and take the tee apart to make sure nothing is restricting it. If it were my truck I would do the following:
Install a disc/drum master cylinder, or a disc/disc master cylinder with a drum appropriate residual pressure valve in the rear brake circuit
Install a proportioning valve to deal with the rear brake lockup problem (also gives you the system failure warning if you hook up a light to the electrical connection), hope that helps.