Air hissing where the lines meet the trailer? Here is what is usually leaking.

Two lines, two different jobs

Between the back of the cab and the nose of the trailer run two air hoses, each ending in a coupling called a glad hand. State CDL manuals describe them the same way. The service line, also called the control or signal line, carries air controlled by the foot brake or the trailer hand valve: it is the signal that tells the trailer brakes to apply and how hard. The emergency line, also called the supply line, fills the trailer's air tanks and holds the trailer's spring brakes released while you drive.

The manuals give the color convention too: blue for the service line and red for the emergency line, and some units also carry metal tags stamped with the names. Each glad hand has a rubber seal on its face, and once two glad hands are rotated together, those seals are all that keeps air in at the joint.

What happens when a line separates

If the emergency line comes apart or is torn off, the trailer loses its supply pressure. The CDL manuals explain that this makes the trailer's emergency brakes come on, and that the tractor protection valve closes to keep air in the tractor's own brake system; you will usually see the red trailer air supply knob pop out.

A service line failure is the sneakier one. With the brakes off, the service line carries no air, so a large leak in it may go unnoticed until you press the brake pedal. Then the air meant for the trailer brakes pours out and tank pressure drops quickly. If it falls far enough, the tractor protection valve closes, which the manuals put in the range of 20 to 45 psi, and the trailer's emergency brakes apply. A service line leak that seems harmless while parked can turn into lost air in the middle of a stop.

Crossed lines look like a leak but are not. With the red and blue glad hands swapped, supply air goes down the service line instead of charging the trailer tanks, and the spring brakes will not release. Our post on brakes that will not release covers that problem.

Why glad hands leak

When the hiss is at the coupling itself, the cause is almost always one of these.

• A worn, torn or missing seal. Glad hand seals are rubber and they wear out. They flatten, split, harden in the cold and get pinched when the couplings are twisted together, and occasionally one simply falls out.
• Dirt, grit or ice on the seal. The manuals tell drivers to clean the couplers and seals before connecting, because a seal that looks fine can leak around a speck of grit. Frozen slush does the same in winter.
• A bent, cracked or worn coupler. Glad hands get dropped, run over and dragged. A bent lip or a cracked casting will not hold the two faces together, and a new seal will not cure it.
• Couplings that do not match or do not lock. A replacement glad hand of a different style, or two worn couplings that no longer grip firmly, can sit together without fully locking, sealing at rest and leaking as the hoses move.
• A loose or cracked fitting. The joint where the hose threads into the glad hand, or into the trailer's bulkhead fitting, can loosen or crack and sound exactly like a seal leak.
• No dummy couplers. The manuals note that dummy couplers keep water and dirt out of disconnected hoses. Glad hands left swinging free collect grit that ends up in the next seal.

Chafed, kinked and pulled hoses

The hoses between tractor and trailer flex every time the rig turns, and they tend to fail in a few predictable ways.

• Rubbing. A hose resting on the catwalk, the deck plate or the frame rail slowly wears through its outer cover. The leak starts as a weep and ends as a split.
• Kinks. Hoses that are too long, or that have lost the spring or hose tender holding them up, fold over when the rig straightens out. A kink restricts air and eventually cracks the hose at the fold.
• Pulled hoses. Lines too short for the combination can be pulled taut in a tight turn, straining the fittings or pulling a glad hand partly open.
• Damage further back. The trailer's own lines run back along the frame to the brakes, and a line rubbing on a crossmember or crushed by road debris can leak well away from the glad hands.

Finding the leak without getting hurt

Get fully onto the shoulder or off the road, turn on your hazard flashers and put out your warning triangles as 49 CFR 392.22 requires; our post on where to place warning triangles covers the distances. Then work through these checks in order.

• Set the brakes and chock the wheels. Parking brakes on and chocks at the trailer wheels before you walk between the units.
• Build full air, then shut the engine off. A leak is far easier to hear with the engine quiet.
• Listen with the trailer supply on. With the red knob in, the emergency line is pressurized all the time, so a leak on that side hisses constantly.
• Check the service side with the brakes applied. The service line only carries air when the brakes are applied. Have a helper hold the pedal while you listen, and agree beforehand that nobody releases a brake or touches the controls while you are near the units.
• Use soapy water, not your hands. A spray of soapy water bubbles where air escapes. Do not feel for leaks with your face or fingers.
• Exhaust the line before you disconnect it. Pull the red trailer supply knob out before separating the emergency glad hands. A pressurized glad hand can snap open hard.
• Stay out from under the trailer. If the leak is somewhere under the trailer rather than at the nose, stop looking. It is not worth crawling under a unit on a shoulder.

Temporary fixes that are not fixes

It is tempting to make a leaking line hold with whatever is in the side box: tape around a split hose, a hose clamp over a crack, a zip tie cinching a coupling, or a seal borrowed from another glad hand. Do not rely on any of them. The service line has to deliver full pressure every time you brake, and a patch that holds at rest can blow off the first time you need the trailer brakes hard. The same goes for swapping the lines round to get a trailer to release.

A leaking brake line is a brake defect, and brake defects are among the first things an inspector looks at. Our post on DOT out-of-service faults covers how inspections treat them, and your carrier's safety department can advise on your unit. The proper repair is quick, so there is rarely a reason to drive on an improvised one.

What the service truck replaces on site

Leaks at the glad hands and in the lines between tractor and trailer are routine roadside repairs. The service truck replaces glad hand seals, glad hands, the hoses between the units and their fittings, and damaged sections of trailer air line; split lines, blown seals and leaking couplings are all on the trailer repair list. Once the parts are in, the system is aired up and checked again for leaks, and the brakes are applied and released before anything moves.

There are honest limits. If the air is not escaping at the couplings or lines, the problem may be a compressor, an air dryer or a cracked tank, and our older post on whether an air leak is a roadside repair or a tow sorts out which of those go to a shop. A line running through a bent crossmember is a structural problem first, and anything that needs the unit on a lift is shop work. When the trailer has to move and its brakes cannot be released safely, a heavy wrecker can take it; whether a trailer can be towed without its tractor explains how.

What to describe when you call

Say whether the leak is on the red line, the blue line or somewhere you cannot pin down, and whether it hisses all the time or only with the brakes applied. Say whether it is at the coupling, along the hose or back under the trailer, whether the rig still builds and holds air, and whether the trailer brakes have applied on their own. And tell us exactly where you are sitting relative to the traffic lane, because that decides how the job is approached before anything else does.

Sources

• 49 CFR 392.22, Emergency signals; stopped commercial motor vehicles

Related reading

Trailer Landing Gear Won't Crank or Has Collapsed — Landing gear that will not crank is usually seized from rust or lack of grease, stripped inside its gearbox, or being asked to lift a load in the wrong gear.

Trailer Lights Not Working? Troubleshoot the Seven-Way — Most trailer light failures start at the connection between tractor and trailer: a corroded or loose seven-way plug, a worn pigtail cord, or a bad ground.

Trailer Brake Dragging or a Hot Hub: Roadside Fix or Tow? — A dragging trailer brake heats one hub, and a hot hub can end in a wheel-off or a fire.