Study guide
4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands
Alberta Air Brake Q Endorsement
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1 Chapter 1: Alberta Q Endorsement Program and Air Brake Fundamentals
1.1 Alberta Q Endorsement Path, Course Requirements, and Knowledge Test Logistics 1.2 Licensing Scope, Farm Vehicle Exceptions, and the Technician Route 1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours
2 Chapter 2: The Supply Circuit: Compressor, Governor, Tanks, and Air Dryers
2.1 Air Compressors, Drive Systems, and Governor Operation 2.2 Supply and Service Tanks, Contamination, and Draining 2.3 Air Dryers, Air Brake Antifreeze, Safety Valves, and Check Valves
3 Chapter 3: Service Circuits, Control Valves, Gauges, and Low-Air Warnings
3.1 Dual Air Systems: Primary and Secondary Service Circuits 3.2 Brake Chambers, Control Valves, Quick Release Valves, and Relay Valves 3.3 Air Pressure Gauges, Low-Air Warnings, and the Driver's Response
4 Chapter 4: The Park Circuit and Tractor/Trailer Circuits
4.1 Spring Parking Brakes, the Park Control Valve, Compounding, and Caging 4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands 4.3 Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers
5 Chapter 5: Air System Failures and Emergency Braking
5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits 5.2 Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks
6 Chapter 6: Foundation Brakes and Brake Adjustment
6.1 Foundation Brakes: S-Cam Drum, Disc, and Wedge Brakes 6.2 Brake Stroke, Adjustment Limits, and Stroke Indicators 6.3 Adjusting Manual Slack Adjusters and Maintaining Automatic Slack Adjusters
7 Chapter 7: Braking Fundamentals, Driving Techniques, and Brake Management Systems
7.1 Braking Fundamentals: Heat, Brake Lag, Stopping Distance, and Fade 7.2 Downgrades, Auxiliary Retarders, Bobtailing, and Winter Braking 7.3 Antilock Brakes, Automatic Traction Control, and Stability Control
8 Chapter 8: Vehicle Inspections and the Air Brake Practical Test
8.1 Trip Inspection Law, Major and Minor Defects, and Roadside Enforcement 8.2 Inspection Set-Up and Recognizing Air Brake Defects 8.3 Trip Air Brake Inspection Steps 1–3: Tractor Protection and Park Control Checks 8.4 Trip Air Brake Inspection Steps 4–6: Supply Circuit, Leak Test, Brake Adjustment, and Scoring
4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands
Key Takeaways Explain This Topic AI
The tractor protection system has two parts: the trailer air supply valve (red button), which controls the supply line, and the tractor protection valve, which uses supply line pressure to open the service line.
The red trailer air supply valve must pop out at 40–60 PSI (276–414 kPa) or higher as tractor pressure is lost.
The supply coupler is marked 'Emergency' or 'E' and conventionally connects on the driver's (left) side; the service coupler is marked 'Service' or 'S'.
In a two-button system, pulling the yellow park control also pops the red button; holding the red button in while pulling yellow sets only the tractor springs, which helps stop trailer linings freezing to the drums.
Inspect glad hand seals and couplers before connecting, carry spare seals (thick side into the coupler), and blow out the lines briefly before connecting to keep dirt out of trailer valves.
Last updated: September 2026
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.
Line Coupler marking Usual side on the trailer What it carries
Supply (emergency) line "Emergency" or a large " E " Left (driver's) side, by convention, not by rule Constant air from the tractor to charge the trailer tanks and hold the trailer spring brakes released
Service line "Service" or a large " S " Passenger side, traditionally The brake application signal from the foot valve or hand valve, present only while the brakes are applied
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
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.
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 :
Button Pulling it... Automatic pop-out
Red (trailer air supply) Applies the trailer spring brakes only, which is the bobtailing mode Yes, on a trailer supply line failure
Yellow (park control) Applies all spring brakes, for maximum park application Yes, on a combined primary and secondary circuit failure
Blue (tractor park) Applies only the tractor spring brakes and leaves the trailer charged, for pre-trips and cold weather No; it is not affected by circuit failures
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:
Leave the tractor's glad hands lying open on the deck.
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.
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.