As the name suggests, the four-circuit protection valve has the job of protecting the individual circuits from each other. It should be prevented that, when in one system there is a pressure loss, other systems are also strongly impaired. Basically, it concerns 4 restrictor valves for the brake circuits 1 and 2 (21, 22) the hand-brake (23) and the operating pressure (24). It would be disastrous if a leak in the operating circuit destroyed the function of the braking system.
The four-circuit protection valve should in addition, also determine the order in which the individual circuits are filled.

The air pressure flows in by way of the compressor, the air-dryer and the pressure control valve from above through the connection '1'. If the system has been purged, the brake circuits above on the left and on the right (figure 3) are first filled. the two check-valves in the center open only after a cetrtain pressure has been reached. Thus sufficient air pressue is achieved in the brake circuits, only then is, e.g., the handbrake circuit filled (figure 4). It is therefore, not possible to drive off with too little brake pressure.

In the further course, the connections on the left and the right are supplied (figure 5). A leak in the operative circuit would only lower the pressure in the remaining circuits to the point where the spill-valve of the operative circuit closes (figure 7). The cross-section dimensions of the valves have been chosen so that, when in continuous operation, more is transported than is lost through the spill-valve in the operative circuit. In the event of a leak in one of the other brake circuits, then the pressure in the other circuits is also lowered. In this case the (adjustable) spill-valve also prevents a complete breakdown. 11/09