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CDL · Air Brake Parts

What is the purpose of the safety valve (pressure relief valve) in the supply tank?

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

To protect the air system from over-pressurization by releasing air if the pressure gets too high (typically around 150 psi)

  1. A To drain the tank
  2. B To protect the air system from over-pressurization by releasing air if the pressure gets too high (typically around 150 psi)
  3. C To start the engine
  4. D To apply the brakes

Why this is the answer

The SAFETY VALVE (pressure relief valve), usually installed in the supply (wet) tank, protects the air brake system from OVER-PRESSURIZATION. CDL air brakes. If the air pressure rises too high (typically the safety valve is set to release at around 150 psi) — for example, if the governor malfunctions and the compressor keeps pumping — the safety valve OPENS and releases (vents) air to prevent dangerously high pressure that could damage the system or cause a tank to rupture. NORMAL operation keeps pressure around 100-125 psi (controlled by the governor); the safety valve is a BACKUP that only activates if pressure gets too high. If the safety valve releases air, it indicates a problem (usually a faulty governor) that needs repair. Knowing the safety valve protects against over-pressurization (~150 psi) is commonly tested.
Source: FMCSA CDL Manual — Air Brakes, Safety Valve

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About this topic: Air Brake Parts

Knowing the parts of the air brake system — compressor, governor, supply tank, dryer, slack adjusters, spring brakes — is required so you can recognize defects during inspection. This is one of 22 Air Brakes questions on air brake parts, 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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