Air Brake Coach
Driving with air brakesQuestion 96 of 116

What happens to the primary service brakes of a bobtail with empty trailer lines?

  1. The brake application is cut by as much as 80 per cent
  2. The application is doubled
  3. There is no change
  4. The brakes lock
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Correct answer: The brake application is cut by as much as 80 per cent

A tractor with a bobtail relay cuts primary service circuit applications by as much as 80 per cent.

Source: Air Brake Manual: A Guide for Instructors (Government of Alberta, Alberta Transportation, March/April 2012), Module 9: Driving tips, page 88

Practice driving with air brakes

Questions last reviewed: 18 October 2026