The tractor protection valve protect the tractors air supply from being depleted in the event of a trailer breakaway.
Any truck or tractor designed to pull a trailer fitted with air brakes will be equipped with a tractor protection system.
The tractor protection system consists of two valves:
1) the tractor protection valve;
2) the trailer air supply valve.
The tractor protection valve is comparable to a sentry.
It monitors air pressure in the trailer.
If the tractor protection valve detects catastrophic air loss in the trailer, it tells the trailer air supply valve to shut off air to the trailer.
The trailer air supply valve turns air on and off to the trailer.
The misconception is that the trailer air supply valve turns the spring brakes on and off on the trailer - that, however, is a subsequent action.
Think of the trailer air supply valve like a light switch.
The light switch controls the flow of electricity and subsequently the light goes of and on.
Like the light switch, the trailer air supply controls airflow to the trailer.
The subsequent action is that it also turns the spring brakes off and on on the trailer.
There are 3 tests to test the tractor protection system:
1) Chock the wheels, release the parking brakes and pump system pressure between 20 and 45psi - the trailer air supply and parking brake control valves on the dash will pop out
2) Go to the back of the trailer and ensure that the brakes are applied.
If the brakes are applied the push rod and slack adjuster will form an approximate 90° angle.
3) Remove the gladhands and make service brake application.
If the tractor protection valve is working there won't be any air loss for either the glandhands on the truck or the trailer.
All the 3 tests indicate that the tractor protection system is working normally.