The Prevost Maintenance Manual specifies that the maximum pressure should be 125 PSI.
Our coaches have many pressure-sensitive components, including brake chambers and air bags. Increasing the pressure to address a problem is like using aspirin to treat a brain tumor—it’s not a proper solution. I’ve heard various justifications for raising air pressure, but so far, every instance has been to fix an issue unrelated to pressure itself. For example, some coaches with excessive weight on the front axle have had pressures increased in an attempt to achieve proper driving height. The issue could be incorrect front air bags (a problem addressed in OKC) or excessive front axle weight.
If the rear doesn’t rise when the tag axle is lifted, it might be due to too much weight in the rear or an issue with the hydraulic system designed to prevent rear drop, or perhaps unrealistic expectations. When the tag is lifted, the four air bags on the drive axle must carry the load. In my bus, they handle 8,000 pounds, which will lift the rear to the driving height, though it requires patience.
If your tag axle isn’t lifting, it could mean the axle and wheels exceed 6,000 pounds, or there’s an issue with the system. Typically, if the tag isn’t designed to be raised, you won’t find a #30 brake chamber just above and behind the tag axle. Depending on the mechanical advantage of the lifting arm, the brake chambers can lift up to 3,000 pounds at 100 PSI each.
Increasing system pressure beyond recommended limits can mask underlying issues. While some converters may ignore axle weight limits and some owners overload their coaches, be cautious with pressures around 135 to 140 PSI. These levels approach the air system’s overpressure limits, which are in place to protect both the bus and its components. Good luck to those of you pushing these limits.