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The brake compensator valve is normally incorporated in vehicles which have front disc brakes and rear drum brakes. Depending on make and model they are usually located adjacent to the brake slave cylinders. On older cars they can be under the car ,mounted in the brake line system that runs between the front and rear brakes.
The brake compensator valve, also known as a proportioning valve, is a small cylinder valve that's incorporated into the hydraulic fluid line of the braking system. The function is to prevent wheel rear locking under heavy,or emergency braking. In the event of heavy braking a piston inside the cylinder moves forwards and partially restricts the flow and pressure to the rear drum brakes and prevents wheel lock.
Indications that there's a fault in the brake compensator valve can be that the rear wheels lock and skid under heavy braking while the front wheels are still revolving. Problems with the valve are often due to a seized piston through corrosion or broken seals.