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Canada · Air brake test

Air brake relay valve and the other valves

An air brake relay valve sits near the rear brake chambers and sends air straight from a tank to them when you press the pedal, so the rear brakes apply and release faster. Without it, air would have to travel all the way from the foot valve, and that delay is brake lag. Nova Scotia’s Air Brake Manual explains it, along with the quick release, check, limiting and modulator valves that keep an air brake system working smoothly.

  • 50%Front braking, slippery road setting
  • 60 psiAbove this, automatic limiting stops
  • Near the rear brake chambersWhere the relay valve sits
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Updated October 2026 · Written from official sources, listed at the bottom of this page

What does an air brake relay valve do?

  1. A larger pipe runs from the tank to the relay valve, and the line from the foot valve becomes a control line that ends there.
  2. When you press the pedal, the control air acts on the top of the relay valve, which passes tank air to the rear chambers at the same pressure as the control air.
  3. Release the pedal and the relay valve exhausts the chambers through its own quick release feature.

On tandem rear axles it speeds up both applying and releasing.

Quick release valve

Without one, air in the brake chambers would have to go back to the foot valve to release the brakes, which is slow on a long wheelbase. A quick release valve near the chambers lets the air out right there, so the brakes release quickly and fully.

Check valves

A one-way check valve between the tanks lets air flow in one direction only, so air can’t flow back toward the compressor. A two-way check valve, between the foot valve and the trailer hand valve, sends the trailer whichever application pressure is higher.

Front brake limiting and modulator valves

  • Manual front brake limiting valve: with the cab control in the slippery road position, front braking is cut to 50% of the rear application pressure.
  • Automatic limiting valve: holds off front braking up to a preset 0 to 10 psi (pounds per square inch), cuts it by about half up to 40 psi, by less than half from 40 to 60 psi, and not at all above 60 psi.
  • Spring brake modulator valve: if the primary circuit fails, it lets air out of the spring brake circuit as you press the pedal, so the spring brakes on the drive axle do the braking.

The control valves in the cab

  • Spring brake control valve: applies and releases the spring brakes (its knob is described on the system page).
  • Tractor parking brake valve, where fitted: normally a round blue knob, releasing the tractor’s spring brakes while the trailer’s stay on.

The tractor parking brake knob is from Ontario’s Official Air Brake Handbook. The trailer supply valve and trailer hand valve are on the trailers page.

Trailer supply and tractor protection valves →Air brake system parts →
Sources: Nova Scotia Registry of Motor Vehicles · Ontario Official Air Brake Handbook and 2 more. Checked October 2026.

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

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