Wouldn't think it'd bethe master cylinder. The usual causes of imbalance is a seized or leaking wheel cylinder or the shoes themselves stuck or glazed up.

Like you say, the master cylinder supplies the pressure equally so that isn't the cause. Maybe there is a fault with the compensator valve, but in my experience, they do not go wrong unlike the old style ones Vauxhall used years ago on the Nova, Corsa and Astra where there was an in-line valve on each rear brake line.

Your valve will be the type with two pipes in, two out and a sprung, load sensing actuator. The piston rod and pivot on these can seize up, but this'll cause both rear brakes to be effected, plus, that itself is an MOT fail if either is indeed seized.

All this is of course is just for the service (foot) brake operation. If there's a problem with the handbrake operation then the fault could be seized cable, seized shoe arm pivots or if both foot and hand brakes effected, leaking wheel cylinder.

You'll need the rear drums off and an inspection of shoes, cylinders, drum faces and cable operation first, then if all's OK there, start looking at the compensation valve.

Good luck with it and report back with what you find.

Hello,

I'm having the same problem with my corsa, and on heavy braking the left rear wheel is locking completely.

Did you solve your problem, what was it, the brake compensator, or the master cylinder?

Can you help?

Regards

Hello

Did you solve your problem, what was the cause of the braking imbalance?

I'm having the same problem but cant figure it out.

Thank you