The filling of the compressed-air system should only be possible up to a pressure (depending on the system) of between 8 and 12 bar. Above that, the compressed air supply should not simply be switched off, rather, the air should be conducted at normal pressure into the environment to save the compressor unnecessary work and to avoid vehicle efficiency losses.
In the above figure, the air coming from the dryer flows in below on the left. With low pressure, only the top left connection (to the compressor) is supplied with compressed air. With higher pressure, the check-valve, below on the right, opens, and the air goes to the four-circuit protection valve. Once the cut-out pressure, set by the screw on top, is reached, the relevant membrane is raised, further below a valve opens which allows a further valve to open the spring-loaded idling valve. The air current now generated by the compressor flows almost pressure-less into the open. The compressor is now idling.
The valve below opens by itself, if, in the system, or in the filters a blockage occurs. Apart from that, it ensures, by means of the check-valve, below on the right, the compressed air supply against a reverse air-flow. Should the tyre inflating connection be used, a check valve, on the top right, also opens and at the same time, blocks the way to the four-circuit protection valve below on the right.