Re: 1967 Camaro Drum Brakes Locking Up ????
I'd replace the master cylinder. If all 4 wheels are locking, my guess is that's where the problem is. We had a customer's car do the same thing,and it turned out to be a faulty O-ring in the master, which would swell when hot,and not allow fluid to return, thereby keeping the brakes applied.
As was mentioned,you should check the length of the booster rod to be sure its not too long, but you need to remove the master to check this anyway, so why not do both? A new master is pretty cheap insurance.
Change the master,and be sure to bench bleed it before installation. My 2 cents.