The master brake cylinder should transfer the pedal force to the hydraulic system. Under normal conditions, this should be done with equal force for both circuits. If one of the two circuits becomes faulty, the task of the other one is to seal the intact circuit against the leaky circuit. In any event, the braking pressure point must be reached before the brake pedal reaches the floor.
The reservoir is, above a certain height, divided. The tandem master brake cylinder has two pistons. The first piston on the right is activated by the piston-rod and thus, by the brake pedal. For this reason it is also known as the 'purh-rod piston', the other one is known as the 'intermediate-piston'. This results in two separate pressure chambers, one between the two pistons and one from the intermediate piston to the rear wall of the master brake cylinder.
Based on the above picture, the pressure chambers are to the left of the piston, whereby, in the course of this article, we'll refer to the right pressure chamber as brake circuit 1 and the left pressure chamber as brake circuit 2. With pistons having the same diameter, at this point it doesn't make any difference, whether the circuits are diagonally divided orwhether they lead to the slave cylinders of the front- or rear axle. Each part of the reservoir is connected by a compensation channel (left) and a supply channel (right) with one of the pressure chambers.
Each of the two pistons has two sealing gaiters, a primary gaiter (on the left) and a secondary gaiter (on the right). Primary gaiters seal off the pressure chambers directly on their left. The secondary gaiter of the push-rod piston seals against the outside and the secondary gaiter of the intermediate piston seals to the pressure chamber of braking circuit 1. For this reason, it is sometimes referred to as a 'separation gaiter'.
The respective compensation channels end, when the brake is inoperative, in the circuits 1 and 2 directly to the left before the primary gaiter. If the brake is being operated the respective primary gaiter passes over the respective supply channel. The pressure built up by the push-rod piston operates the intermediate piston. Possible springs or other connections are, during the pressure build up, of no consequence.
The pressure balance between the two brake circuits is guaranteed solely by the pressure between the two shifted intermediate pistons. The pre-condition for this is of course, that both pistons have the same diameter. There are however, systems where both parts of the main brake circuit and the respective pistons are deliberately differently sized. 03/12