During a braking procedure in an electronically controlled compressed air brake system for pulling vehicles, which is only partially schematically depicted, a module 13, according to DE 42 26 697 C1 and having a control piston jointly associated with the electrical control circuit 25 and the pneumatic control circuit 26, controls a pneumatic control pressure air that is conveyed by way of a pneumatic control line 38 to a pressure relief location 3 in the module 13 during a braking procedure. The control electronics 45 modulate the pneumatic brake pressure request signal conveyed by way of the control line 38, for example, as a function of the load or braking slip, in the following manner High Quality Trailer Control Valve:
When a pressure increase is conveyed to the control input 59 of the trailer control valve 14 as a result of the occurrence of an electrical brake pressure request signal conveyed by the control circuit 25, the inlet valve 48 switches into its open position. When a pressure decrease is conveyed to the control input 66 as a result of the occurrence of a pneumatic brake pressure request signal conveyed from the control circuit 26 of the service brake system 11, the outlet valve 47 switches into its closed position.
In the event of a failure of the electrical control circuit, a second control chamber with a control piston is kept in reserve in the trailer control valve, and this is used only when the electrical control circuit is functional again. This is achieved by means of a mechanism, which keeps the control piston disposed in the second control chamber mobile during operation of the electrical control circuit, so that it can again be used for generating a pneumatic control pressure.
