Two Leak Tests That Both Fail
You are running the pre-trip air check on a 3-axle tractor and a 2-axle van trailer. With the system fully charged to 120 psi, engine off, wheels chocked, and parking brakes released, the gauge falls to 113 psi in 60 seconds. With the brake pedal held down, it falls from 118 to 108 psi in 60 seconds. After you start the engine, the compressor builds from 85 to 100 psi in about 45 seconds, then cuts in again roughly every 90 seconds at idle. The trailer ABS lamp comes on with the ignition and stays on; the tractor ABS lamp goes out a few seconds after start-up. During your walkaround you hear a faint hiss at the trailer relay valve that appears only when the service brakes are applied.
1. Interpret both static leak tests and decide whether this combination moves.
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Both tests fail. With the system charged, engine off, parking brakes released, a combination may lose no more than 4 psi in one minute; yours lost 7, from 120 to 113. With the service brakes applied and held, the limit for a combination is 6 psi per minute; yours lost 10, from 118 to 108.
That is not gauge flutter or rounding. It is a real leak, and the second test tells you where. The loss roughly doubles when the pedal is down, and you hear the hiss at the trailer relay valve only with the service brakes applied. So the leak is on the service (application) side, at or downstream of the trailer relay valve.
Do not dispatch. A service-side leak means the trailer's brakes apply weakly and late, the compressor runs continuously, and on a long grade the trailer does not contribute its share of braking. That imbalance is exactly what pushes a combination into a jackknife, and it also means the trailer may not hold on a grade when you stop.
Keep the trailer spring brakes set or chock the wheels, get the relay valve and its fittings repaired, then re-run both tests at 120 psi before you move.
2. The trailer ABS lamp stays on after the ignition check. Does that mean the trailer has no brakes? What do you do, and how does it change the way you drive?
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No. The trailer still has brakes. ABS is an add-on that modulates pressure during a wheel-lock event; when the lamp stays on, the ECU has disabled ABS on that trailer and the brakes work conventionally, without antilock protection.
What you lose is the ability to keep the trailer wheels rolling under hard braking on slick pavement. Locked trailer wheels give no steering authority from that axle and greatly increase the chance the trailer comes around; in a combination, the trailer locking first is the classic start of a jackknife.
What to do:
- Confirm the lamp actually lit at ignition and stayed on, since a dead bulb is its own defect.
- Check the ABS electrical cord, the trailer ABS connector, and the wheel sensors for corrosion or damage; that fixes a fair share of these on the road.
- If it stays on, the trailer can generally be moved but should be repaired at the first opportunity. Check your state's rules, since some treat an illuminated trailer ABS lamp as an out-of-service item.
Drive accordingly: longer following distance, lower speed on wet roads, no hard brake applications, and no pumping the pedal.
3. The compressor cycles about every 90 seconds at idle. What does that indicate, and why does it matter on top of the static leak tests?
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A compressor cycling every 90 seconds at idle is working hard. Once the system is charged, an idling combination should normally sit quietly for several minutes before the compressor cuts back in.
Three things cause rapid cycling: a leak (most likely here, given the static tests), a failed unloader or governor, or a compressor that can no longer keep up.
The safety consequence is the same either way. The system never sits at full pressure, so you have less reserve. Each brake application spends air, and if the compressor is already running almost continuously, the gauge will fall on a grade and may not recover. Add a service-side leak at the trailer relay valve and the trailer's brakes also apply weakly, so the tractor does most of the work with less air behind it.
Taken together: a 7 psi/min release leak, a 10 psi/min applied leak, and rapid cycling lead to one conclusion. The leak must be found and fixed, not driven around. Re-test at 120 psi afterward: 4 psi/min or better with brakes released and 6 psi/min or better applied before a loaded run.