I bought an adjustable valve from Speedway Motors similar to this one, http://www.speedwaymotors.com/Speedway-Purple-Proportioning-Valve,23498.html . Its installed at the back of the truck.

Mine did the same as yours, so I took it to the local open parking lot and after about five, 40 mph stops, I had it dialed in. It does make for a heavier brake pedal, but the truck stops straight and I know what it's going to do if I ever really have to stand on the brakes.

Joe