Air Brake Coach
System partsQuestion 14 of 116

Which relay valves have higher crack pressures?

  1. Relay valves at the rear of the vehicle
  2. All relay valves have the same crack pressure
  3. Relay valves closer to the cab
  4. Only trailer relay valves
Show answer

Correct answer: Relay valves closer to the cab

Higher crack pressure gives the control air more time to reach the rear before the power unit brakes apply.

Source: Air Brake Manual: A Guide for Instructors (Government of Alberta, Alberta Transportation, March/April 2012), Module 4: Service circuits, page 53

Practice system parts

Questions last reviewed: 18 October 2026