The trailer system is supplied by blended tractor air taken from the primary and secondary circuit through a two way check valve as previously described.

The system is charged by opening the trailer supply valve (24), allowing air from the tractor to pass through the tractor protection valve (23) and the trailer spring brake valve (30) directly into the spring brake chambers (18). When air enters, the pressure protection part of the trailer spring brake valve opens, allowing the air to fill the trailer tanks.

When a brake application is made, blended control air acts on the relay valve (14) that releases air from the trailer tank to the brake chambers.

If the tractor breaks away from the trailer, the service and supply lines will be pulled apart. The sudden loss of air in the supply line will cause the tractor protection valve to close, preventing air from escaping out of either broken connection. The air supply in the tractor is sealed off and is available to control the tractor brakes. At the same instant, the sudden loss of air in the supply line causes the trailer spring brake valve to exhaust the air from the trailer spring brake chambers applying the trailer brakes. The trailer brakes cannot be released under these conditions unless the lines are recoupled and the trailer tanks recharged.

If only the supply line breaks between tractor and trailer, the same sequence of events will occur.

A break or rupture in the service line will not affect the trailer until a brake application is made. A loss of pressure in the tractor system will then result, causing the same emergency brake application described above. However, the driver will be able to release the parking brakes by releasing the foot valve, rebuilding air pressure and opening the trailer supply valve.

To apply the parking brakes, the control valve (19) is closed, causing a loss of pressure in the line that applies the spring brakes as described above.

The old and new tractor and trailer system are fully interchangeable, whether they are dual system or single circuit and whether they are systems with spring brakes or without.

NOTE: An air dryer (28) has been added to reduce the amount of moisture in the system.