Rear disk conversion issue

I have a weird issue with my 64 D100 project. I have scarebird on front and rightstuff on rear. I have adjusted the parking brake. I used a tool to adjust the booster pin and it is not touching. There is no residual pressure valve. I have adjusted the proportioning for full pressure at rear. The master is a ssbc corvette style with booster made for a 69 charger. I’ve had this issue twice now that the rear brakes are dragging. I stopped during a 1.5 hour trip yesterday and the rears were hot. I first noticed the smell. The rear use the gm type calipers with the external parking brake. I completely released the parking brake and turned the pressure almost off to the rear. When I got home the rears were hot again. I’m a little lost. Any suggestions? Could the master or booster fail in a way to be engaging the rears? Craig