Figure 7-50.- Combined-limiting and quick-release valve.

Figure 7-51.- Tractor protection valve and switch.

The tractor protection system functions as a set of remotely controlled cutout valves (fig. 7-52). The trailer service and emergency lines pass through the valve. When the control valve is in the NORMAL position, service and emergency braking functions of both the tractor and trailer are normal. When the valve lever is in the EMERGENCY position, the trailer air brakes lines are closed off.

Should acondition resulting in severe air loss from the tractor or trailer air brake system be detected or if for any other reason it is desirable to cause an emergency application of the trailer brakes, the operator can move the control valve lever to the EMERGENCY position. At this time both the trailer service and emergency brake line will be closed off at the tractor protection valve. Such operation offers a convenient daily check of the relay emergency valve on the trailer where tractors and trailers are not

Figure 7-52.- Tractor protection valve piping.

disconnected over long periods of time. The operator should move the control to the EMERGENCY position when disconnecting a trailer or when operating a tractor without a trailer if cut-off valves are not installed in the trailer connections on the tractor. The tractor protection valve should NOT be used as a parking brake, because it was not designed for that purpose.