Need help diagnosing a braking problem.

Wise gentlemen and ladies of the forum,

Project Scarlett the “scumbag scamp” is off to a start. New front hubs bearings and some steering refresh have it back in the road and doing apocalyptic burnouts. However. I’m having a braking problem I’m having trouble diagnosing and haven’t been able to find on the forum.

I step on the brake and everything is fine till a certain brake pressure. Then the brakes seem to apply themselves all the way to a panic stop whether My foot is on the pedal or not. It sucks the pedal down and then holds the brakes if it’s own accord I till judicious throttle is applied or a combination of that and pulling the pedal back up with my foot. Front system discs have been entirely rebuilt with the exception of calipers. Booster looks new ish and master does as well, which is full and no leaks at any of the wheels. Drums look crusty but functional with no obviously leaking wheel cylinders. Any thoughts?