Losing air pressure: stop while you still choose where
Stop. If the air warning is on and the gauge is falling, you are not deciding whether to stop, only where — and the window in which that is still your decision is short. Below a threshold the vehicle sets the spring brakes itself, without asking, wherever it is at that moment. Get onto a hard standing while you still have service brakes and steering, then find the leak with the engine off. Most air leaks are audible in thirty seconds of standing still and silent, and a good half of them are fixable on the spot.
Finding it
A leak that is inaudible at idle is usually audible with the engine off, and one that is inaudible at rest often appears the moment the brake pedal is held down. Both checks take under a minute and between them they find most of what goes wrong.
- Park on firm ground, chock a wheel, engine off, and stand still for thirty seconds. Listen before you look.
- Work along the obvious first: the gladhand couplings between tractor and trailer, then the air lines where they run along the chassis and rub.
- Have someone hold the brake pedal down while you listen at the brake chambers. A diaphragm that only leaks under application will hide from every other test.
- Check the suspension bellows. A split bellows sounds like a slow hiss and looks like a corner sitting lower than the other three.
- If you have soapy water, use it on anything you suspect. If you do not, a wet finger works on a small leak.
- Note where it is and whether it is on the tractor or the trailer before you call. Those are different parts on the van.
Why it happened, and why it will happen again
Air systems rarely fail suddenly. What usually happens is that a small leak has been there for weeks, the compressor has been covering it, and something — a cold night, a rough section, an extra tonne — tips the balance so that it can no longer keep up. That is why a truck that was fine yesterday can empty in twenty minutes today.
The single most common underlying cause is a saturated air dryer. Once the cartridge stops removing moisture, water goes through the system: it corrodes valve seats, it sits in the tanks, and in winter it freezes in exactly the narrow passages where it does most harm. A dryer cartridge is a cheap part with a service interval that quietly gets skipped, and it is behind a surprising share of the calls we get on the first hard frost.
Related
The compressor keeps up if I keep the revs up. Can I drive on like that?
No, and the reason is that the balance you are relying on is not stable. A leak grows, the compressor does not, and every stop at a junction is a moment when the revs drop and the margin disappears. You also cannot see the gauge and the road at the same time for the whole distance. Every driver who has ended up with the spring brakes set in a live lane got there by deciding this exact question the other way.
Can I just drop the trailer and drive the unit to you?
If the leak is definitely on the trailer and you are somewhere a trailer can legally and safely be left, sometimes yes — but that is a decision to make on the phone, not alone on a hard shoulder. A trailer parked in the wrong place is its own problem, and a leak that appeared to be on the trailer is on the tractor often enough that arriving without the trailer can mean the fault is no longer reproducible. We come out; that is the point of the number.
Truck stopped right now?
A mechanic answers, day or night, and you get an arrival time before you hang up.