CDL Air Brakes TestAir Brake System PartsHard
A technician is inspecting the air brake system of a commercial truck and observes that the safety relief valve on an air tank is continuously releasing air, even though the supply pressure gauges indicate approximately 130 psi. What is the most likely cause of this issue?
- AA blocked air dryer causing excessive moisture build-up.
- BAn improperly adjusted slack adjuster.
- CA leak in the service brake chambers.
- DA faulty air compressor governor failing to cut out the compressor.
Show answer & explanationAnswer & explanation
Correct answer: D. A faulty air compressor governor failing to cut out the compressor.
The safety relief valve opens to prevent catastrophic over-pressurization of the air tanks, typically when pressure exceeds 150 psi. If it's releasing air at 130 psi, and continuously, it indicates that the air compressor is still pumping air beyond its normal cut-out pressure (usually 125 psi) because the governor has failed to signal it to stop.
Why the other options are wrong
- A. A blocked air dryer affects air quality and moisture removal, not system over-pressurization to this extent.
- B. Slack adjusters affect brake application, not the primary system pressure or governor function.
- C. Leaks in brake chambers would cause pressure to drop, leading to the compressor cycling more, not over-pressurization.
Safety Valve Activation
The opening of the safety relief valve on an air tank to release excessive air pressure, protecting the system from over-pressurization and potential rupture.
- Typically set to open at 150 psi.
- Indicates a failure in the air compressor governor or unloader valves.
- Continuous activation is a serious safety concern.
- Requires immediate diagnosis and repair of the compressor/governor system.
Memory trick: Hissing valve, pressure so high, governor failed, a dangerous sigh.