When I got my 68 P1800 the dual circuit master cylinder was leaking from behind but the brakes functioned as they should with no pulling or sticking. I then changed the master cylinder with a newly rebuilt one and I went ahead and changed the brake shoes in the rear drums, the front calipers and brake pads as well. I then bled the entire system. The problem I am having now is that the rear brakes seem to lock up when I stop at a light and hold on to the brakes. When I let them go and drive I can feel the rear brakes hold on and then let go suddenly. What could it be? I have checked the rear drums for leaks and kinked lines, but nothing. i have also rebled the system using a Might Vac as well as the prescribed bottle method and still nothing. Any ideas?