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

[Xeen]

My first thought and the easiest way to solve the problem is to buy an adjustable proportion valve, if that doesnt work you will have no choice but to buy a Rear Disk Big Brake Conversion Kit and slap it on because I just dont think the factory drums just are up to the task and no matter what you try they never will be with that much load. This will require a Rim swap to a 16" or larger.
You could also just try a standard rear disk conversion anything is better than those drums although honestly I think the big brake conversion is best for that application.
If you have to do the Big Brake conversion brace yourself because this isnt going to be cheap.

[Wrenchbender Ret]

Those are plenty big brakes. What are you useing for booster. Those large brakes came with Hydroboost.

[airdale94]

Also are your rubber brake hoses new? I'm not sure what the press. are supposed to be but that doesn't sound like much.

[RichardJ]

Non-adjustable proportion valves are designed for specific applications. I doubt yours is working correctly, but the master could be faulty.

In the chart below for one specific application, 900 psi before the prop valve, should only be reduced to about 700 psi after the prop valve.
Did you check the pressure before the prop valve? Pressure coming out of the master and into the prop valve should be the same as the pressure coming out of the master and to the front brakes.

I.E. in this case with 900 psi going to the front brakes, you should also have 900psi going into the prop valve.