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Canada · Air brake test

Air brakes on trailers

Air brakes on trailers connect to the truck or tractor by two lines: a supply (emergency) line that fills the trailer’s tanks and a service line that carries braking pressure. The trailer has its own tanks and spring brakes, which apply by themselves if the supply line loses pressure. Valves on the towing vehicle make sure a problem on the trailer doesn’t drain the tractor’s air.

  • 2Air lines between the tractor and trailer
  • 20 to 45 psiWhen the trailer supply valve closes itself
  • Below 60 psiWhen trailer spring brakes apply
Where to study

Updated October 2026 · Written from official sources, listed at the bottom of this page

How do air brakes on trailers connect to the truck?

  • Each line joins with a pair of metal couplers called glad hands, with a rubber seal in each.
  • You put them together at an angle and rotate them until they lock.
  • In most cases they’re colour-coded, red for supply and blue for service, and most are shaped so they only fit the matching coupler.
  • If the lines are crossed, the trailer brakes won’t work properly: the spring brakes may not release and the service brakes won’t work, so it isn’t safe to drive.

Trailer supply valve and tractor protection valve

Ontario’s handbook describes the trailer supply valve, near the driver:

  • Push it in to send air to the trailer; pull it out to shut it off.
  • If air escapes or the trailer breaks away, it closes by itself when supply line pressure falls to between 20 and 45 psi (pounds per square inch).
  • When it closes, the tractor protection valve closes too, so the towing vehicle keeps its air.
  • In an emergency, if the trailer service brakes fail, pulling the supply valve applies the trailer spring brakes.

Trailer spring brakes and priority

Ontario’s handbook says trailer spring brakes must apply automatically when supply line pressure drops below 60 psi, whether you closed the supply valve or the line broke. Some trailers fill the spring brake tank first (spring brake priority) and can roll before the service brakes have enough air; others fill the service brake tank first. Ask your employer or a qualified person which one your trailer uses.

Trailer hand valve

A hand valve on the steering column or dash applies the trailer service brakes on their own. If you use it and the pedal together, the trailer gets the higher pressure. Never use it to park or for an emergency stop.

Testing trailer air brakes

The daily check includes two trailer tests: the tractor protection valve test and the automatic application of the trailer spring brakes. Ontario’s handbook has a circuit diagram of trailer brakes.

The air brake pre-trip inspection →Ontario handbook: Trailer brake circuit diagrams ↗

The licence you need to pull an air-brake trailer

Not every class can tow a trailer with air brakes. The licence classes page sets out Ontario’s Class A rules and Alberta’s and New Brunswick’s Class 3 limits.

Licence classes and air brakes →
Sources: Ontario Official Air Brake Handbook · Ontario Ministry of Transportation · Alberta.ca and 3 more. Checked October 2026.

Rules and fees can change, so check the official site before your test.

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