function of the tandem master brake cylinder is to control both brake circuits. Thus, the braking power of one circuit is not affected if the other one breaks down. The pressure-point must still be achieved despite the longer pedal travel, before the pedal reaches the floor. This is why, under normal braking circumstances, a pressure build-up must also occur, even with a short amount of pedal travel. The brake should also completely release, this means, both pistons must also return to their initial positions.
When in the release position, both pistons are near to the compensation ports. In addition, these have a port which is tapered toward the cylinder. This is because their diameter and also the distance that the release-point is from the compensation ports, influences the amount of free pedal travel before the braking actually occurs.
If the brake does not release properly, it's possible that the there is too little play between the push rod piston and the push rod.
When in the braking position, there is a hydraulic connection only between the two pistons. This ensures an equal pressure distribution between the two brake circuits. Thereby, depending on the brake circuit layout, the piston push-rod (on the right) travels about twice the distance that the floating piston (on the left) does.
If the piston push-rod returns to it's initial position because no braking force is being applied, the floating piston, through a shackle- screw and sleeve is also returned. This mechanical connection ensures that, despite the two piston-springs, both pistons are in position, just in front of their compensation ports and ready for the next braking operation.
If the brake does no longer release after a longer braking operation, the assumption can be made, that the compensation hole is blocked.
In the event that the first brake circuit fails, there is also a mechanical connection between the two pistons. If there is a leak in the second circuit, the floating piston and it's extension are shifted against the wall of the main brake cylinder. In both cases, the defective circuit is emptied and there is still enough brake fluid to build up the pressure in the intact circuit. 03/12