brake problem

[nra4life]

Greetings, I seem to have outsmarted myself. '72 C/20. '90 14 bolt single wheel rear end. It has 13" x 3 1/2" drum brakes. My front brakes are '87 1 ton van calipers on the '72 rotors. The caliper bores are 3 3/32". They are bigger than the originals. The master cylinder was changed from 1 1/4" to 1 1/8" to raise the line pressure. My pressure to the front is 900 lb. The rears are 300 lb. With a 13,000 lb. rv 5th wheel trailer, it won't stop. Shouldn't the line pressures be at least double what I'm getting? If so, how do get the pressures up? I am baffled!! Roy