Question about brakes grabbing

[BP68]

Hey all,

I have a 68 with but my cab, brake pedal, booster came off a 72 that had power brakes. The master cylinder came from my buddy who got it new for his 71 with power brakes from CPP I think. It has been a while.

Anyway, when I first started driving the truck, the rear was locking up bad. Found out I had a leaking alxe seal. Got that fixed.

Brakes work great from inital push to about just over 3/4 of the way down.
But once I hit that last 1/4 to bottom of travel, brakes seem to lock up. I think it is the fronts locking.

So press pedal and feels right till right before end of travel than the grab hard. Any ideas on what could cause this. Could the wrong prob valve cause this? The prop valve is supposed to be a disc/drum unit, but it was one my bodyman had and installed, so I cannot be 100% sure. Just seems weird all is good till right when you reach end of pedal travel.

thx for any help/suggestions.