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.

Relay Emergency Valve

The relay emergency valve (fig. 7-53) acts as a relay station to speed up the application and release of trailer brakes. It automatically applies the trailer brakes when the emergency line of the trailer is broken, disconnected, or otherwise vented to the atmosphere if the trailer air brake system is charged. It is used on trailers that require an emergency brake application upon breakaway from the truck or tractor.

When a tractor is connected to a trailer and the service and emergency lines are opened, the relay emergency valve permits charging the trailer air brake reservoir to approximately the same air pressure as that in the tractor reservoirs. During normal operation of a tractor-trailer unit, the relay emergency valve functions as a relay valve and synchronizes trailer service brake air pressure and tractor service brake air pressure, as the treadle valve on the tractor is operated. The trailer brakes can also be applied independently of the tractor brakes by use of the hand control on the tractor protection valve on the tractor and the relay emergency valve on the trailer.

If a trailer is disconnected from a tractor for loading or unloading or if the trailer is separated from the tractor under emergency breakaway conditions or if the emergency line of the trailer is vented to the atmosphere by other means, the relay emergency valve applies the trailer brakes. This is automatically achieved by using the existing trailer reservoir air pressure. If the trailer is to remain parked under these conditions, the wheels should be blocked to prevent the possibility of a runaway.

If you are required to release the emergency brake application on a trailer under these conditions, the trailer reservoir drain valve can be opened or the trailer air brake system can be recharged through the trailer emergency line.

You can check the relay emergency valve by moving the tractor protection valve control lever to the EMERGENCY position, if tractor protection equipment is installed. If no tractor protection is