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 falls outside these numbers, something needs attention.
The 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 tests the supply side.
- Apply and hold full brake pedal. After the first drop, watch the gauges for another minute. This tests the service side.
- Compare 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
- 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 by road brine
- 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 visible. Keep the wheels chocked, and never reach near chambers or slack adjusters while someone could apply or release the brakes.
Winter: frozen lines and valves
On the coldest East Tennessee mornings, air problems aren’t always leaks. Water that gets past a tired air dryer can freeze in lines, valves, and gladhands, so brakes won’t release or pressure won’t reach the trailer.
- Drain the tanks regularly, and service the air dryer before winter.
- Don’t use an open flame to thaw air lines or valves.
- Use only air-system antifreeze products your truck’s manufacturer approves, and use them sparingly — some can damage seals.
- If the brakes won’t release after a cold night, call. Forcing it can damage 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, never cage spring brakes to keep going, and never start down a long grade with an air problem.
A leak the compressor can keep up with may still let the truck move, but it keeps the compressor working all the time, 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.
General information only. This guide is written for drivers and dispatchers working around Knoxville and East Tennessee. Rules change and every truck is different — follow your carrier's procedures, current FMCSA regulations, and the directions of the Tennessee Highway Patrol or other officers on scene.