What normal looks like
Every truck is a little different, so check your manual, but these are the standard benchmarks taught in 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 automatically 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 falls outside these numbers, something needs attention.
The leak-down test
- Park on level ground and chock the wheels. Build air pressure to governor cut-out.
- Shut the engine off and release the parking brakes.
- Watch the gauges for one minute and note how much pressure drops. This tests the supply side.
- Apply and hold full brake pedal pressure. After the initial drop, watch the gauges for another minute. This tests the service side.
- Compare the results to 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 chafed where they rub the catwalk or frame
- Air 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
- Spring brake chambers leaking from the spring side
- Air-powered 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 valves 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 visible — bubbles form where air escapes. Keep the wheels chocked, and never reach near brake chambers or slack adjusters while someone could apply or release the brakes.
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 means the compressor runs constantly, the air dryer is overloaded, and the truck may not release its brakes after sitting. It will also likely fail an inspection. Get it fixed before it strands you.
Why air leaks are so common in harbor work
Drayage is unusually hard on air systems, for three reasons that all compound each other.
- Hook-and-drop volume. A harbor tractor connects and disconnects gladhands many times a day, every day. Seals that would last months in linehaul work wear out in weeks here.
- Pool chassis. You did not own that chassis yesterday and you will not own it tomorrow. Its air lines, gladhands, and brake chambers are somebody else’s deferred maintenance until the moment they become yours. Check them on the hook, not at the gate.
- Salt air. Coastal corrosion attacks fittings, valve bodies, and the threads on everything. A lot of harbor air leaks are at connections that look fine until you put a wrench on them.
Add long idles in gate queues, where a small leak quietly bleeds the system down while you wait, and you get the most common call we take: a truck that built air fine this morning and will not release its brakes now.
Gladhand seals cost almost nothing. Carry a handful of them and a spare air line, and check the chassis before you pull out of the terminal rather than after.
General information only. This guide is written for drivers and dispatchers working the Port of Los Angeles and Port of Long Beach. Regulations change and every truck is different — when in doubt, follow your carrier's procedures, the current FMCSA and CARB regulations, the terminal operator's rules, and the instructions of law enforcement on scene.