Make sure you check the front brakes too. I had a similar problem
with my MGB. The right rear would lock up under heavy braking, with old
tires. With new tires, I noticed a pull to the right, but no lock up.
The problem was a leaking left front caliper contaminating the brake
pad. Poor braking at the left front for some reason caused the right rear
to take up the slack and lock first, probably something to do with
ionization and cosmic radiation.
Since everyone is already thinking of brakes... Rear drivers side brake
(drum)is kinda on or off. It is the first to lock, even prematurely. Car
(MGA) stops straight and sufficiently. Noticable at very low speeds.
Without undo pedal pressure, that wheel will suddenly lock. I've taken
brakes off and re-assembled, cleaning and lubing appropriate areas with
proper brake backing plate lubricant. 1 yr old MC, but same symtoms with
old MC. Brake bleeds okay, but have not removed and inspected wheel
cylinder. Seems that the wheel cylinder is the only thing left. Have I
answered my own question?
86 Audi 4kq