Rupture of the compressor discharge line would result in loss of pressure from the wet tank. When the pressure in the wet tank (2) of the tractor unit falls below the warning level, due to a compressor failure or excessive leakage on the tractor, the warning devices (11) will start to operate. In the diagram, the one-one check valve (9) has prevented the main reservoir air in the dry tank from escaping back to the wet tank and the ruptured line.

There is sufficient reservoir air pressure in the dry tank for a limited number of brake applications to stop the vehicle before the parking brakes are activated. This will depend on how the parking brakes are piped.

If this pressure is lowered to approximately 45-20 P.S.I. the tractor protection system will close automatically, placing the trailer brakes into an emergency position.

The tractor protection system described in this manual is an example of a vehicle equipped with a type of cab mounted trailer supply valve (24) that will close automatically when the pressure in the supply line drops below 45-20 P.S.I. The valve may also be closed manually.