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CDL · Warning Devices

What does the front brake limiting valve do?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

On older vehicles, reduces braking force to the front axle on slippery roads when set to 'slippery'

  1. A On older vehicles, reduces braking force to the front axle on slippery roads when set to 'slippery'
  2. B Prevents the front brakes from working at high speeds
  3. C Disconnects the brakes when towing
  4. D Limits the number of times the brake pedal can be used

Why this is the answer

Older trucks (made before 1975 or so) sometimes had a front brake limiting valve that the driver could set to 'normal' or 'slippery'. When set to 'slippery', the valve reduced air pressure to the front brakes, the idea being to prevent the front wheels from locking on slick surfaces and losing steering. In practice, studies showed that limiting valves caused more harm than good — they reduced overall stopping power without delivering the intended safety benefit — and they have been removed from modern vehicles. If you drive an older truck with a limiting valve, leave it in 'normal' for all conditions. The valve is not a substitute for modern ABS or appropriate driving for conditions.
Source: FMCSA CDL Manual 5.1.8 Front Brake Limiting Valve

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About this topic: Warning Devices

The low-air warning gives you seconds, not minutes, before spring brakes apply automatically. Knowing what the warnings mean — and what to do when you see them — is essential. This is one of 4 Air Brakes questions on warning devices, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

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