check return spring on combo pedal. Also the micro switches in column. If one sticks, it wont work. Look at the inching cylinder with floorboards off. If the plunger doesn't remain in the pressure position it will not move.
you can try to replace the trans filters, but me thinks you are going to get a new/rebuild trans... you are NOT really getting a good clutch pack lock up with those pressures. the book calls for about 140 to 170 psi in the main, about 105 to 125 in the rev and forward test ports.
might be related to the inch valve linkage motion/lack of complete stroke...
this truck may be a single brake pedal and not a separate inch and brake pedal.
Are the symptoms the same (losing power) if you use the inching pedal?