Study guide
4.3 Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers
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
4.3 Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers
Key Takeaways Explain This Topic AI
The foot valve applies both tractor and trailer service brakes; the trailer hand valve applies only the trailer service brakes, through the service (blue) line.
Trailer hand valves are mounted on the steering column or dash; some spring back to released and others stay applied until released, but none may be used for parking.
Tank-mounted relay valves on trailers use the service signal to deliver trailer tank air directly to the trailer chambers.
Spring park brakes have been required on trailers since January 1, 1976 (CMVSS 121); air park brakes remain on most converter dollies and some heavy-haul equipment.
An air park brake application is about twice as powerful as a spring park application, but it releases as air leaks away, so trailers with air park brakes must be chocked when uncoupled.
Last updated: September 2026
Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers
Quick Answer: Pressing the foot valve applies the tractor and trailer service brakes together. The trailer hand valve sends control air through the service (blue) line to apply only the trailer service brakes. It is never a parking brake, because it holds the trailer with air that will leak away. Trailer relay valves , usually mounted in the trailer tank, turn that service signal into a fast, full-volume application from the trailer's own tank. Trailers built since January 1, 1976 must have spring park brakes . Older air park brake systems, still used on most converter dollies , are about twice as powerful as spring brakes but release as the air drains , so those trailers must be chocked when uncoupled.
Two ways to send a service signal to the trailer
Control Where it is What it applies
Foot valve Cab floor; the valve itself may be inside the firewall, outside it, or under the floor Tractor and trailer service brakes
Trailer hand valve Steering column or dash Trailer service brakes only
Both controls feed the trailer service line through a double check valve , which passes whichever signal is higher . That signal goes through the tractor protection valve and the service line to the tops of the trailer relays. If you use both at once, the trailer receives the stronger of the two signals.
Types of hand valve: some are spring-loaded and return to the released position unless the driver holds them. Others stay applied until the driver manually releases them.
Why the hand valve is never a parking brake
A hand valve application holds the trailer with air in the service chambers . Air leaks away past seals and fittings over time, and as it does the brakes release, with nothing to warn you. The Alberta course states plainly that hand valves are not to be used for parking . To park, apply the spring brakes with the dash controls and chock the wheels when required.
Using the hand valve correctly
The Commercial Driver's Guide warns against using the trailer brakes too much . It leads to more wear on the trailer brakes than on the tractor brakes, unbalanced braking between tractor and trailer, and a combination that could jackknife . Legitimate uses in the Alberta materials include:
Brake response check: the final step of the trip air brake inspection is a brake response test with the foot valve and another with the trailer hand valve .
Tractor protection check: with the lines disconnected and the red button popped out, you can apply and hold the foot valve or the hand valve and confirm that no air leaks from the open service line.
Blowing out the lines before connecting glad hands (Section 4.2).
When you use the leak test to check the whole system, remember the one gap in it: the only line not filled during a service application is the control line from the hand valve to the double check valve (on vehicles that have hand valves).
How the trailer relay valve works
On a trailer, the relay valve is typically mounted directly in the service tank :
Service function: the small service-line signal arrives at the top of the relay, and the relay delivers trailer tank air straight to the trailer chambers . This avoids the lag of pushing the whole application volume back from the tractor. On release, the chamber air exhausts at the trailer.
Emergency function: when supply line pressure is lost , because of a breakaway, a ruptured supply line, or the driver pulling the red button, the trailer's emergency brakes apply. On a spring brake trailer the spring brakes apply.
Long trailer service lines weaken the signal. Converters and some trailers use a zero-crack-pressure booster relay to rebuild it (Section 3.2).
Air park brake trailers
Before 1976 , trailers had air park brakes instead of spring brakes. Since January 1, 1976 , Section 121 of the Canadian Motor Vehicle Safety Standards (CMVSS 121) has required spring park brakes on trailers . The older system is still used on most converter dollies and on some specialized heavy-haul equipment, so you may still meet it.
Feature Spring park brake trailer Air park brake trailer (and most converter dollies)
How the park or emergency brake applies Springs apply when supply air is exhausted Air from the trailer service tank is sent into the service chambers
Strength About half of a full service application A very powerful emergency brake, about twice as powerful as spring park brakes , with a higher risk of lock-up in an emergency application
Does it stay on? Yes, mechanically No. As air drains out of the system, the park brakes release
What pushing the red button does Charges the tanks and releases the springs Releases the air park brakes and maintains air in the trailer's supply tank
The communications to both kinds of trailer are the same, so any tractor can pull either one . The operating procedures are very different:
Chock an uncoupled air park brake trailer , because its brakes will release as the air leaks away.
Before coupling , charge the trailer with air and apply its service or park brakes so that it cannot roll.
Watch for reversed lines on this kind of trailer. Section 5.2 explains why reversal can leave it with no service or park brakes.