CDL Air Brakes TestAir Brake System PartsHard
A driver observes that the supply pressure gauge for the primary air tank reads 120 psi, but the secondary air tank gauge reads only 80 psi. The air compressor is running normally. What is the most likely cause?
- AA malfunctioning air compressor governor.
- BA stuck-open safety valve in the primary tank.
- CA leak in the secondary air tank system.
- DA clogged air dryer.
Show answer & explanationAnswer & explanation
Correct answer: C. A leak in the secondary air tank system.
In a dual air brake system, if the compressor is running and the primary tank is at full pressure (120 psi), but the secondary tank is significantly lower (80 psi), it indicates that air is not being retained in the secondary system. This is most commonly due to a leak in the secondary air tank itself or its associated lines/valves.
Why the other options are wrong
- A. A malfunctioning governor would affect both tanks, either preventing them from reaching full pressure or causing over-pressurization.
- B. A stuck-open safety valve in the primary tank would prevent the primary tank from reaching 120 psi, and likely the secondary tank as well.
- D. A clogged air dryer would affect the quality of air (moisture/oil) and potentially slow down charging, but wouldn't cause one tank to hold pressure while the other doesn't once charged.
Dual Air Brake System
A dual air brake system consists of two separate air brake systems: one for the front axle(s) and one for the rear axle(s), each with its own air tank(s), lines, and often a gauge. A check valve prevents air loss from one system if the other fails.
- Provides a safety redundancy.
- Each system usually has its own gauge.
- One system can operate even if the other fails.
Memory trick: Two tanks, two gauges; if one's low, check its ways.