Some vehicles are equipped with a different type of cab mounted trailer supply valve, which must be operated manually by the driver. It has two positions: NORMAL and EMERGENCY.

The tractor will be equipped with a tractor protection valve, and the trailer unit with a relay emergency valve, as in the previous system.

The functions of the trailer supply valve, tractor protection valve and the relay emergency valve will be similar to those illustrated. An important difference is that the trailer supply valve must be shifted to the EMERGENCY position manually.

CHARGING THE TRAILER SYSTEM:

The driver places the trailer supply valve in the NORMAL position and main reservoir air will be directed to the trailer reservoir.

TRAILER BREAKAWAY:

The supply line will cause the trailer brakes to dynamite. To prevent air loss from the tractor, the driver must shift the trailer supply valve to the EMERGENCY position. Otherwise the tractor air pressure will bleed down and hold at 45-20 P.S.I.

RUPTURED SERVICE LINE:

As in the example, no action will occur until a brake application is made. Application of the brakes with a ruptured service line will result in a loss of pressure in the system. When the main reservoir pressure drops to between 45-20 P.S.I., the trailer brakes will dynamite.

RUPTURED SUPPLY LINE:

As in the example, the loss of supply pressure will cause the trailer brakes to dynamite. To prevent air loss from the tractor the driver must shift the cab-mounted trailer supply valve to EMERGENCY. Otherwise the tractor air pressure will bleed down and hold at 45-20 P.S.I.

If a slow loss of main reservoir air occurs, when the supply line pressure drops to between 45-20 P.S.I., the relay emergency valve will cause the trailer brakes to dynamite.

MANUAL DYNAMITING OF TRAILER:

Any time the driver shifts the cab-mounted trailer supply valve to the emergency position, and the trailer system is charged, the trailer supply valve will exhaust the supply line, dynamiting the trailer brakes.

The trailer brakes will remain applied only as long as air pressure remains within the trailer system. How long the air in the system will hold the brakes applied depends on how airtight the system is. As a safety precaution, parked trailers should always be blocked to prevent a possible runaway.

To move a trailer that has been parked with the brakes in an emergency application it is necessary to charge the system to release the trailer brakes.