Author: Jill McBeth
Date: May 13, 2024

Roadcheck 2024: Focusing on Tractor Protection Systems

What is a Tractor Protection System? And what does it do?

The tractor protection system is like a parachute. If the trailer becomes separated or the air system fails, the trailer brakes engage so the trailer doesn’t continue traveling like a missile and kills people. The tractor protection system works with the air system to control the trailer. When the air supply to the trailer is cut off, the tractor protection valve closes, which activates and applies the brakes on the trailer.

The tractor protection system has 2 valves:

The trailer supply valve (TSV)

The trailer supply valve (TSV) is a a push/pull red button on the dashboard and the glad hand on the trailer is also red.

The other is the tractor protection valve (TPV)

The tractor protection valve (TPV) is a a push/pull yellow button on the dashboard. The TPV is also called a ‘towing vehicle’ protection valve and the glad hand on the trailer is blue.

When there is no trailer, the driver opens (pushes in) the tractor supply valve (TSV). When a trailer is being towed the driver opens the trailer supply valve (TSV) (pushes in). The trailer supply valve (TSV) can also be used to set the trailer parking brakes.

In my experience doing level 1 inspections, the Roadside Inspection/Test Procedures for the Tractor Protection System can be a difficult concept to explain to some drivers. Most drivers learn this in school and never use it again until they get pulled over for a level 1. It’s a good idea to have a refresher in Roadside Inspection/Test Procedures in real life not just a email to all the fleet before Roadcheck. Any super truckers in your fleet should be teaching this, leverage that knowledge and experience. 

Conducting the tractor protection system test

The goal of this test is to confirm that the Towing Vehicle Protection System is functioning properly in the event of a trailer separation. This test is done under pressure with the engine off and is conducted as follows:

trailer supply valve TSV – red button and tractor protection valve TPV – yellow button.

1. The system must be working at normal operating.

2. Wheels chocked and all brakes must be released, push the red and yellow buttons on the dash in.

3. Remove gladhand couplings to simulate a trailer separation. Driver will remove the gladhands and put them on the deck.

4. Air will start to escape immediately from the gladhand line. In most cases the air will stop escaping immediately and in some cases the air will escape and then stop.  The tractor protection valve TPV should close once the gauge pressure (on the dashboard) drops to 20 psi (138 kPa). If the tractor protection valve TPV fails to close before pressure gets below 20 psi (138 kPa), the power unit is defective and that is an OOS condition.

5. When the gladhands are disconnected, the trailer emergency (spring) brakes must also apply automatically. If the trailer emergency (spring) brakes do not engage the trailer breakaway system is defective. The trailer is in an OOS condition.

6. When the gladhands are disconnected, the trailer system must close automatically. Check the gladhand on the trailer for air bleeding back. Air leaking from the gladhand on the trailer indicates that the trailer spring brake control valve is defective. The trailer has a violation but, not an OOS. 

7. When the air has stopped the driver will apply the brakes and hold it. 

8. Any air escaping from either gladhand line the Towing Vehicle Protection System is defective. This is an OOS condition.

9. If there is air escaping from either the supply/emergency or service/control gladhand line of any vehicle equipped to tow an airbrake equipped trailer indicates the unit has a defective tractor protection system and is an OOS condition.


Take the time to refresh your driver’s skills with this inspection process because it is very easy to get an Out of Service for a air leak.