Pls help brakes lock at bottom of pedal travel

[BP68]

Hey all.
I am having a brake issue. I have a 68 C10 with a 72 factory air cab, using booster and bracket off 72 truck. Master cylinder was new from CPP for a 70 model with power disc/drum. Brakes feel good as you press pedal. As pedal reaches near bottom of travel there is almost a faint sense of bottom, but if you press pedal till it reaches full bottom (maybe just under last 1/4 of movement, brakes lock up.

So a light foot brakes fell great, but as you come to full stop, if pedal is fully pressed till it stops moving there is lock up. Any ideas? It is as if brake pedal bottomed out just under a 1/4" less everything would be great.

Pls help