airbrakecompressorA healthy truck air system spends its life inside a narrow pressure band. The governor loads the compressor at cut-in (about 100 psi / 6.9 bar) and unloads it at cut-out (about 120-125 psi / 8.3-8.6 bar); the low-air warning trips near 60 psi / 4.1 bar, and the reservoir safety valve vents around 150 psi / 10.3 bar. The converter turns any pressure between the two systems, and the table lists the values a technician actually watches for.
psi ↔ bar converter
= ( 827 kPa )
1 psi = 0.0689 bar = 6.895 kPa. Type a pressure to convert.
Standard air brake pressures
| Point | psi | bar | What happens |
|---|---|---|---|
| Low-air warning | ~60 | ~4.1 | Buzzer and dash light; below this, braking reserve is unsafe |
| Cut-in | ~100 | ~6.9 | Governor loads the compressor; it starts building air |
| Fully charged | ~120 | ~8.3 | Top of the normal operating band |
| Cut-out | ~120-125 | ~8.3-8.6 | Governor unloads the compressor; building stops |
| Safety (pop-off) valve | ~150 | ~10.3 | Reservoir relief valve vents excess pressure |
Reservoir, timing and warning-device requirements are set by FMVSS 121 (49 CFR 571.121). These are general reference values for common systems — the exact figures for a given vehicle are set by its manufacturer's service data.
Why the band matters
The gap between cut-in and cut-out, the differential, is normally about 20-25 psi. Too small a gap and the compressor short-cycles and overheats; too wide and pressure sags between cycles. A system that will not reach cut-out, or that trips the low-air warning in normal use, has a fault upstream — often the governor, the unloader, a leak, or the compressor itself. For diagnosis start with the governor cut-in/cut-out guide and the won't-build-pressure guide.
Need the part, not just the answer?
OE-grade air brake compressors and repair kits, manufactured and tested to commercial-vehicle standards.
Shop VADEN partsPublished by VADEN Original. Product links point to the manufacturer’s official catalogue. Specifications are general — always confirm figures against your vehicle’s service manual.