the highway mode highway supply line hsl is activated and connected to passages 58 and 128 which provides a pressure on the bottom of diaphragm 126 for a preselected pressure for example 25 p s i plunger 124 is raised closing the passage 130 of the plunger against the check valve this cuts off the communication between the output 104 of the relay spool valve 102 and the brake cylinder passage 120 upon the pressure rising above the predetermined pressure the plunger continues its upward movement opening the check valve 124 thereby interconnecting the brake cylinder and the brake cylinder passage 120 to exhaust port 140 thus when the vehicle is connected for highway use as detected by brake cylinder release valve 122 the relay spool valve 102 is disconnected from controlling the brake cylinder and the brake cylinder is connected to exhaust thereby deactivating the brake cylinder this prevents the brake cylinder from operating while in the highway mode this is important since if the brake cylinder was locked in the highway mode even though the rail wheels are not on the ground severe damage may occur to the rail wheels if they should hit the ground and not be free to spin with the brake cylinder release valve 122 in the position illustrated in fig2 the output 104 of the self lapping release spool valve 102 is effectively connected to the brake cylinder passage 120 when the control valve 22 senses a brake pressure reduction it responds by generating a control signal cv to passage 74 of the relay valve assembly 28 this signal on diaphragm 76 drives.