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 around Laredo
Drayage and yard work are hard on air systems. Trailers get hooked and unhooked many times a day, so gladhand seals and supply lines wear quickly. Tractors sit in long lines and yards, so small leaks bleed the system down while they wait. And summer heat dries out rubber seals, diaphragms, and hoses. Gladhand seals are cheap — carry spares.
General information only. This guide is written for drivers and dispatchers in the Laredo area. Regulations change and every truck is different — when in doubt, follow your carrier's procedures, the current FMCSA regulations, and the instructions of law enforcement on scene.