After swapping axles on my CJ-7 (D44 front, Ford 9" rear) I could never seem to balance the vacum assist brake system. When I used the factory proportioning valve the front disc's would work great but the rears would hardly apply. I installed a new proportioning valve and the opposite happened. Rears would lock up and fronts wouldn't apply. I then tried an adjustable proportioning valve to reduce pressure to the rear brakes. Even after installing the adjustable valve the front brakes hardly applied. I had no idea what was going on. Totally confused, I removed the proportioning valve completly and just ran straight lines directly from the Wagoneer master cylinder to the front and rear brakes. I maintained the adjustable proportioning valve and also added a 10 pound residue valve to the rear brakes. PERFECT! The brakes have never worked better. In reading this forum it appears that brake bias is a common issue. IMO proportioning valves are now highly over rated.