A tractor protection valve (Fig. 38) is usually mounted on the cab or chassis of the tractor.

When the trailer-supply valve is open, air passes through the bottom of the tractor protection valve and charges the trailer through the supply line (also called the emergency line).

When the pressure in the supply line reaches 45 psi, the service line port of the tractor protection valve opens. This allows application air pressure to travel down the service line to the trailer when a brake application is made.

Note:

• The supply line always contains the same air pressure as is in the highest-pressure circuit (provided the trailer-supply valve is open).
• The service line only contains air pressure when a brake application is made and the trailer-supply valve is open.
• When you are not hooked to a trailer, the trailer-supply valve is closed and there will be no air to the tractor protection valve. Spring pressure closes the service line port. This action protects the application air pressure in the truck.
• On a trailer breakaway, air will rush out of the supply line until the trailer-supply valve automatically closes (automatic type). This prevents any more loss of air from the tractor.