[siggyfreud]

I think the booster might be your problem then. Let's work our way back from the brakes. I am writing it this way not because I don't think you understand, but just because this is how I think about the problem since I am not a brake expert.

Given enough pressure, the brakes will lock up, so we know you're simply not getting enough force at either the front or rear. So if we start at the calipers/wheel cylinders as the last point where pressure is applied and think about what would prevent the pressure for making it there:

1. Are the cylinders or calipers leaking? Doesn't seem like it, and if they were you'd still be able to lock up something I'd imagine, even for a second before line pressure bled off.
2. Are the flex lines leaking, or expanding. Probably not unless you feel extra movement in the pedal. And even then, you'd think that unless the leak was major you'd get at least SOME of the brakes to lock up.
3. Is there leakage in the hard lines somewhere?
4. Is the prop valve set right? - Well your situation doesn't change based on that, and it should. If nothing else, you should be able to get the rears locked up by having the valve wide open. But you said that's not happening.
5. Is the master cylinder right for a disc/drum setup?
6. Is the master cylinder undersized and not moving enough fluid?
7. Is the booster adequately providing enough assist?

To me, a 7" single stage seems really small for a disc/drum setup. I'd think that you'd want a 10-11" single, or 9" double perhaps for something with disc. I think 9" single is what drum/drum setups usually want.

So I think either your master is wrong/undersized on bore size, or your booster is just too small to give proper assist for a disc/drum setup.