I have a 1988 GMC Sierra with approximately 109,000 miles. The problem that I am encountering is that in the snow my truck does not stop completely. It will only stop when you shift it into neutral. If you shift it back into drive the vehicle starts to move again. The brake system has been looked at and is in working order. The master cylinder has been replaced, new whell cylinders in the back, new drums, pads with spring kit. The tourque converter has been checked as well as the entire transmission. Everything was in perfect working order. The only other thing that I can think might be wrong would be the portitioning valve. I need help on figuring out if this might be the next step to take.