What normal looks like
Every truck is a little different, so check your manual, but these are the standard benchmarks from CDL air brake training:
| Check | Typical standard |
|---|---|
| Governor cut-out | Compressor stops pumping at about 125 psi |
| Governor cut-in | Compressor starts pumping again at about 100 psi |
| Air build-up (dual air system) | 85 to 100 psi within 45 seconds |
| Low air warning | Comes on before pressure drops below 60 psi |
| Spring brakes | Apply on their own at roughly 20–45 psi |
| Leak rate, brakes released, engine off | No more than 2 psi/min (single vehicle) or 3 psi/min (combination) |
| Leak rate, brakes applied, engine off | No more than 3 psi/min (single vehicle) or 4 psi/min (combination) |
If your truck is outside these numbers, something needs attention.
Running a leak-down test
- Park on level ground and chock the wheels. Build air to governor cut-out.
- Shut the engine off and release the parking brakes.
- Watch the gauges for one minute and note the drop. This checks the supply side.
- Apply and hold full brake pedal. After the initial drop, watch for another minute. This checks the service side.
- Compare with the limits above, and listen carefully the whole time.
If pressure drops too fast with the brakes released, look at the supply side. If it only leaks with the pedal down, look at the service side.
Where leaks usually hide
Leaking with the brakes released (supply side)
- Gladhand seals — cracked, missing, or flattened seals are the most common trailer leak
- Trailer supply lines rubbed through where they touch the catwalk or frame
- Tank drain valves and tank fittings
- An air dryer purge valve stuck open — you’ll hear constant hissing from the dryer
- Push-to-connect fittings loosened by vibration or corroded over time
- Spring brake chambers leaking from the spring side
- Air accessories: seats, horns, fan clutch, suspension, and sliding fifth wheel
Leaking only with the brakes applied (service side)
- Service brake chamber diaphragms
- The foot (treadle) valve
- Relay and quick release valves
- Service lines to the chambers
- The trailer service (blue) line and gladhand
Tip: soapy water in a spray bottle makes small leaks show up. Keep the wheels chocked, and never reach near chambers or slack adjusters while someone could apply or release the brakes.
Cold snaps: frozen lines and valves
Little Rock doesn’t get many hard freezes, which is exactly why they catch trucks off guard. Arkansas humidity puts plenty of water into air systems, and if a tired air dryer lets it through, that water can freeze in lines, valves, and gladhands on the rare cold morning — brakes won’t release or pressure won’t reach the trailer.
- Drain the tanks regularly and service the air dryer on schedule — humid summers load it up.
- Never use an open flame to thaw air lines or valves.
- Use only air-system antifreeze products your truck’s manufacturer approves, and sparingly — some damage seals.
- If the brakes won’t release after a cold night, call. Forcing it can break parts.
When to stop driving
Stop and call if the low air warning comes on, if pressure won’t build past the warning level, if the compressor can’t keep up with the leak while driving, or if the spring brakes start dragging. Never keep driving with low air, and never cage spring brakes to keep going.
A leak the compressor can keep up with may still let the truck move, but it keeps the compressor running constantly, overloads the air dryer, and may keep the brakes from releasing after the truck sits. It will also likely fail an inspection. Get it fixed before it strands you.
For general information only. This guide is written for drivers and dispatchers working in and around Little Rock and Central Arkansas. Regulations change and every truck is different — follow your carrier's procedures, current FMCSA rules, and instructions from the Arkansas Highway Police, Arkansas State Police, or any officer on scene.