Whats does the REAR BRAKE COMPENSATOR DO?

Its job is to stop the rear wheels locking under heavy braking so it shuts the brake fluid supply off and transfers the braking effort to the front . If the rear brakes locked up under heavy breaking you may skid and loose control of the vehicle .

There is a plate and spring on the valve attached to the rear beam axle [on the passenger side] , the more weight in the rear of the car the longer the valve stays open . smaller engined MK1's 1100, 1300 and some 1500 models didn't have one .



Cabriolet and later than about 1983 models used a pair of valves mounted on the master cylinder [these are still available from VW but are very expensive]
that do the same job as the earlier rear beam mounted item .

If you suspect that the rear beam compensator is faulty then replacement  is the only option , luckily   Car Parts & Spares -- Buy Online with Fast UK Delivery | GSF Car Parts still sell these items .