Air Brake Leakage Test: Applied and Released

The air brake leakage test is two separate checks. With the system charged to governor cut-out (about 120-135 psi on most trucks) and the engine shut off, hold the brakes released and time one minute: pressure should not fall more than roughly 2 psi on a single vehicle or 3 psi on a combination. Then hold the service brakes applied with at least about 90 psi in the tanks and time another minute: the allowance opens to roughly 3 psi and 4 psi respectively. Exceed either figure and the vehicle fails — and the difference between the two readings tells you which half of the system is bleeding down.

Why there are two tests

A truck air brake system is pneumatic, and it has two pressurized zones that come and go with the pedal. With your foot off the pedal, only the supply side is live: compressor discharge line, air dryer, wet tank, service reservoirs, the governor signal line, check valves, the dash control valves and — on a combination — the trailer supply line and the park side of every spring brake chamber. Press the pedal and a second zone comes up on top of that: the treadle valve delivery circuits, relay valves, the quick release valve, service lines and service gladhands, ABS modulator valves and the service side of each brake chamber diaphragm.

So the released test audits the supply side alone. The applied test audits the supply side plus the service side. Two numbers, one subtraction, and you have narrowed the hunt before you ever crawl under the truck.

Before you start

• Chock the wheels. You are about to release the parking brakes with the engine off.
• Build to cut-out and let the air dryer purge. A purge during your timed minute looks like a 4-6 psi loss and will fail a healthy truck. Wait for the pop, then begin.
• Give the air a minute or two to settle. Freshly compressed air is hot, and cooling air loses pressure on its own with no leak involved.
• Key on, engine off, so both dash gauges read.
• Watch a clock with a second hand and read both the primary and secondary gauges. The larger drop is your result.

Released (static) leakage test

1. Charge the system to governor cut-out, shut the engine down, and turn the key back to the on position.
2. Push both dash knobs in — the yellow park control and, on a tractor-trailer, the red trailer supply — so the parking brakes are released and the trailer is charged and included in the test.
3. Keep your foot off the brake pedal.
4. Let the needles settle, note the reading, and time exactly one minute.
5. Record the loss on each gauge.

Applied leakage test

1. From the same charged, engine-off condition, confirm you still have at least about 90 psi. If you are below that, restart, rebuild to cut-out and shut down again.
2. Apply the service brakes with firm, steady foot pressure and hold. Do not pump and do not fan the pedal.
3. Let the reading stabilize — the first few seconds always show the drop that filled the chambers, which is normal and is not leakage.
4. Note the reading, time one minute, and record the loss on both gauges.

Bracing the pedal with a stick or having a helper hold it is fine. Fanning the pedal to "keep it at 90" invalidates the test, because every reapplication dumps air through the relay and chamber circuit and hides a real leak inside normal usage.

Allowable leakage

These are the numbers used in CDL testing and by roadside inspectors in North America. Separate from the leak rate, the regulations require the system to be free of audible leaks, so a hiss you can hear is a defect even when the gauge holds inside the limit.

Reading the result

Chasing the leak

Work in a quiet bay with the engine off and the system charged. Listen first, then confirm with soapy water. For a released-test failure, walk the supply side: drain valves and their bodies, the dryer purge valve, the governor, the compressor discharge fitting, and every gladhand and coupling. For an applied-test failure, have someone hold the pedal down while you listen at each relay valve exhaust port and each brake chamber — a continuous hiss from a relay exhaust with the pedal applied usually means a failed diaphragm or delivery seat, and a hiss at a chamber means a torn diaphragm or a cracked clamp band. Systematic technique matters more than a good ear here, and the same discipline that finds a one-minute failure is what you need for a system that only goes flat overnight; there is a step-by-step method in our guide on finding an air leak fast.

Most of what shows up on this test is a wear item rather than a broken casting: chamber diaphragms, valve seals, gladhand seals and dryer purge valve kits. Match replacements to the OE part number stamped on the valve or chamber you are working on — VADEN's OE-grade air brake system parts are listed by OE cross-reference for that reason. Retest after every repair, because a large leak masks the smaller one behind it.

Mistakes that fake the result

• Timing through an air dryer purge cycle.
• Starting the applied test below about 90 psi.
• Leaving the red trailer supply knob pulled out, which cuts the trailer out of the test entirely.
• Pumping or fanning the pedal instead of holding steady pressure.
• Reading only one gauge on a dual circuit system.
• Blaming a leak for what is actually a lazy or inaccurate dash gauge.

Where it fits in the full check

The leakage test is the middle of the pre-trip air brake routine, not the whole of it. Before it you charge to cut-out and confirm the governor cycles; after it you keep bleeding pressure down to verify the low air warning comes on around 60 psi and that the spring brakes apply on their own somewhere between roughly 20 and 45 psi. Done in that order in the seven-step air brake check, the leak test also gives you a baseline number to write down, and a truck that used to lose 1 psi and now loses 3 is telling you something long before it fails an inspection.