Hy tony b, you say you remove the master cylinder to get the pressure to go away, do you physically remove the master or just crack the lines? As others have said an increase in pressure can be caused when the master cylinder cannot return to its fully released position, either by mechanical linkage binding or improper adjustment of the pushrod. I assume that a truck of this size has power brakes, the only other thing I can think of is a sticking atmosphere valve in the power brake booster, not knowing what kind of booster you have the only suggestion I can make is to replace the booster,hope that helps.
