A tractor protection system is added to prevent total loss of air from the tractor if the trailer breaks away, or if the connecting air lines between tractor and trailer become separated or ruptured. The tractor protection system consists of two valves added to the tractor system: the TRACTOR PROTECTION VALVE (23), and the TRAILER SUPPLY VALVE (24). Other names for the trailer supply valve are ‘trailer parking control’ and ’emergency valve’.

There are two types of trailer supply valves in use. The most common is a spring loaded valve that, after being opened manually, is held open by air system pressure. This device is called an automatic trailer supply valve.

Some tractors have a manual trailer supply valve. These may be a toggle type switch or a push/pull type.

The tractor protection valves vary in outward appearance, depending on make and model, but all perform the same function: protecting the tractor air supply.