Trailer control valve for tractor with electronically controlled brake system

The trailer control valve (6) has a first control piston (25) acted on by the braking pressure in a first braking circuit and coupled to a second control piston (2) acted on by the braking pressure in a second braking circuit, a combined entry and exit valve (19,20,21) operated via both pistons. The surface of the second piston acted on by the braking pressure defines a control chamber (33,35) provided as 2 partial chambers, one of which can be separated from the braking circuit braking pressure.