The word that is used is 'redundance'. What is meant is, how often a system which is relevant for safety, should be duplicated. In the case of the compressed air brake, there is the footbrake with two brake circuits plus a handbrake which is independent of the others. As a rule there is also a third brake. In addition, the handbrake is thus constructed, that one cannot drive off untill the correct pressure has been built up.
Actually it's really quite simple. A very strong spring (shown in red, on the right of the picture) causes the brake-shoes, in the case of drum-brakes, to be pressed towards the drums, with disc-brakes, the pads are pressed onto the discs. Should there be no brake pressure or it has been bled off, the axle with a compressed air reservoir remains in braking position.
In the driving position, the spring must be compressed through compressed air. The diaphragm cylinder on the left, is then responsible for the operation of the footbrake. By the way, the spring and the brake pressure may not act together on the braking mechanism. This is prevented by the pneumatic control. The screw on the right has a special meaning for the mechanic. In the event that the vehicle has to be towed and the handbrake cannot be released because there is no brake pressure. This screw must also be turned in completely when installing the reservoir. 08/08