Painted jester, thanks for that info.
Many years ago I had exactly the same problem on a T-bucket, Buick front discs and master cylinder. It would start dragging even if we were just towing the car. For the life of me I couldn't figure out what it was at the time (20 years ago).
At the race track I disabled the front brakes and ran just using the emergency brake...couldn't get the pedal to stay up after I put the front brakes out of action.
I am almost 100% certain it was because I didn't put the prop valve in right.
Thankfully no one got hurt.