Managing Excessive Air Pressure in Heavy Equipment Systems

[MikePhua]

Understanding the Role of Air Pressure Systems
In commercial trucks and heavy equipment, air pressure systems are critical for multiple operations—most notably, braking. These systems typically include compressors, air dryers, tanks, and valves designed to maintain optimal pressure levels, usually between 90 and 125 PSI depending on the application.
Excessive air pressure, however, can compromise safety, damage components, and signal malfunctioning regulators or faulty compressors.
Symptoms of Overpressure and Its Consequences
Operators might notice air gauges spiking beyond 130 PSI, the pop-off valve releasing frequently, or in worst cases, air lines bursting. One common indicator is when the air governor fails to unload the compressor at its designated cut-out pressure, allowing pressure to build continuously.
In trucks and equipment with spring brake systems, this can also lead to unintentional brake drag or even failure if not properly relieved. An over-pressurized system may also cause air dryers to work harder than necessary, leading to premature failure of desiccants or check valves.
Causes of Excessive Air Pressure

1. Faulty Air Governor
   The air governor regulates the compressor cut-in and cut-out pressures. A stuck or worn-out governor may fail to signal the compressor to stop compressing air, allowing the system to pressurize beyond safe limits.
   
2. Stuck Unloader Valve
   The unloader valve in the compressor is responsible for venting air when the desired pressure is reached. If it fails, the compressor continues working past safe thresholds.
   
3. Blocked or Faulty Safety Valves
   Safety or "pop-off" valves should relieve excess pressure. A clogged valve can delay or prevent this, creating potential for line rupture or damage to air components.
   
4. Misadjusted Air Dryer Systems
   Some air dryers integrate pressure control components. If tampered with or replaced incorrectly, the entire system can lose its balance.
   

Fixing the Problem
The most common first step is replacing the air governor. It's inexpensive and easily accessible on most vehicles. Ensure the replacement matches OEM pressure specifications—typically a cut-in at 100 PSI and cut-out at 125 PSI. If replacing the governor doesn’t solve the issue, the compressor’s unloader mechanism should be inspected.
Experienced mechanics often carry a spare air governor due to its common failure rate and the relatively simple installation process—usually two bolts and two air lines.
A Lesson from the Field
In one notable incident, a vocational dump truck was flagged during a routine DOT inspection for carrying over 140 PSI in its tanks. Upon investigation, it was found that an aftermarket air governor was installed with incorrect specs, leading to the over-pressurization. The issue was corrected on-site, but the driver was issued a fine due to the potential safety hazard.
Regulations and Safety Standards
In North America, FMVSS 121 regulates air brake systems, including maximum allowable air pressure. Systems must include functioning pressure relief devices and warning indicators. Operators are encouraged to perform pre-trip inspections that include monitoring air pressure buildup and ensuring that safety valves are operational.
Conclusion
Excessive air pressure in heavy equipment or trucks isn’t just a mechanical issue—it’s a safety concern. Timely identification and repair of faulty air governors or relief valves can prevent serious incidents. Proper maintenance of air systems and routine inspections are critical for staying within regulatory compliance and ensuring long-term equipment reliability.