I've had a 1 ton with front and rear disc brakes and no troubles as you describe.

I used the common longish brass GM proportioning valve/distribution block.
I believe they make them for trucks with disc and drums as well as a different one for disc and disc.
Perhaps if you have one already it's not the right one?

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-