4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands

The Tractor Protection System, Trailer Supply Valve, and Glad Hands

Quick Answer: A trailer receives air through a supply (emergency) line and a service line. The tractor protection system has two parts. The trailer air supply valve (red button) controls the supply line. The tractor protection valve uses supply line pressure to hold the service line open. If trailer air is lost, the red button pops out at 40–60 PSI (276–414 kPa) or higher. The supply line is then exhausted, the trailer brakes apply, and the tractor protection valve closes the service line so the tractor keeps its air. Glad hands must be inspected, fitted with good seals, and connected securely.

Trailers have no supply circuit of their own

Service and park brakes work on trailers exactly as they do on power units, with relays and control lines as well. The difference is that air comes from the towing vehicle, so a trailer has no supply circuit. Two air lines connect the tractor and trailer and carry both the air supply and the brake signal to the trailer's service and park circuits.

In the Alberta schematics the two lines are drawn red (supply) and blue (service) to match the lines on real trucks. The air that charges the trailer comes from whichever tractor service tank has the higher pressure.

The two parts of the tractor protection system

1. Trailer air supply valve (red button). This is the driver's control for the supply line.
   • Push in: charges the trailer tanks and releases the trailer's spring brakes. When you push it in after coupling, the tractor gauges drop as the trailer fills. That drop is normal and expected. A tridem trailer with empty tanks and air-ride suspension can pull the tractor's pressure down substantially.
   • Pull out: exhausts the supply line, so the trailer's brakes apply and the tractor protection valve closes.
   • Automatic pop-out: it pops out on its own at 40–60 PSI (276–414 kPa) or higher as pressure is lost.
2. Tractor protection valve. This valve uses supply line pressure to open the service line. When the supply line loses pressure, the tractor protection valve closes the service line, so a service application no longer blows tractor air out of a broken or disconnected line.

The trailer supply valve is checked by enforcement officers at roadside inspections, and a vehicle whose tractor protection system does not work is placed out of service. In a Level 1 CVSA inspection, the officer removes both lines from the trailer. When the red button trips, the driver is asked to apply the service brake, and no air should leak from the service line. You perform the same check in the trip air brake inspection (Section 8.3).

Two-button and three-button dash controls

Two-button systems (current equipment). Pulling the yellow park control valve applies all of the vehicle's spring brakes, and the red trailer supply valve automatically pops out with it. Many drivers hold a thumb on the red button while pulling the yellow one. This applies only the tractor's spring brakes and keeps the trailer spring brakes released, which prevents the trailer brake linings from freezing to the drums in cold weather.

Three-button systems (older equipment). These have a red trailer supply valve (usually on the left), a yellow park control valve (usually in the middle), and a round blue non-automatic tractor park control:

All three must be pushed in to release every spring brake. The course notes that three-button systems are not used in modern equipment.

Glad hands: inspect, clean, connect

Inspect before connecting. Glad hand couplers should be completely inspected before they are connected:

• Seals that are soft or degraded will leak. Carry spare seals and know how to replace them: the thick side goes into the glad hand coupler.
• Couplers that are damaged may not press together properly and will leak.

Keep dirt out of trailer valves. Trailer air valves are expensive. Dirt picked up in the lines while bobtailing can migrate into them and cause costly repairs. The course's "Road Sense Tip" is to blow the lines out quickly before connecting:

1. Leave the tractor's glad hands lying open on the deck.
2. Pull the trailer hand valve and hold the red dash valve in for a few seconds. Air blows through both lines and carries dirt out.
3. Release the dash valve and the hand valve, then connect the lines.

Connect and secure:

• Glad hands that are polarized and in good shape cannot be connected in reverse.
• After connecting, confirm the couplers are properly locked. Check that the lines are properly suspended so they will not catch as the trailer turns.
• A straight truck set up to pull a pony or full trailer should have its glad hands on dead-end (dummy) couplers when no trailer is attached.
• Coiled plastic lines work well but become brittle over time. Rubber lines are also common. Flat-deck haulers sometimes run the lines from the back of the tractor frame to the landing-gear area of the trailer so that loads overhanging the front of the deck do not interfere with them.