Slow Leaks on Duals: Stems, Extensions and Rims
The inside dual that is always a little low is usually a stem, extension or rim, not the tire. How to find a slow leak and fix it before it blows out.
Every fleet has one. The inside dual on the trailer that is twenty pounds down every week, gets aired up, and is twenty pounds down again the next time anyone checks. It never goes flat, so it never gets fixed, and one day on a hot afternoon on I-10 it is the tire that lets go. Slow leaks on duals are the single most preventable tire failure on a working truck, and most of them are not in the tire at all.
Here is where slow leaks actually come from and how to find and fix them.
The valve stem
The valve core itself, the rubber or metal stem, and the seal where the stem meets the wheel. Cores loosen, get gummed up, or corrode. Rubber stems crack at the base after years of heat. Metal stems on tubeless truck wheels seal with a grommet that hardens and leaks. On an inside dual, the stem is hard to reach, so it gets bumped with the air chuck and bent, and a bent stem leaks at the base.
The test is soapy water. Spray the stem, the core, and the base and watch for bubbles. A leaking core is a thirty-second fix. A stem is a dismount job, but a short one.
The extension
Valve extensions on inside duals are convenient and they leak. The threaded connection loosens, the seal inside the extension fails, or the extension gets caught on something and bends. An extension that is not properly supported flexes with every rotation and works itself loose. If a dual has an extension and a slow leak, check the extension first. Tighten it, soap it, and if it still bubbles, replace it or remove it and air the tire the hard way.
An extension that leaks is worse than no extension. It lets the inside dual go low quietly, and a low inside dual is what overloads its mate and kills both tires.
The rim
Steel truck wheels rust at the bead seat. Aluminum wheels corrode and pit there. Either way the tire bead sits on a rough surface and air seeps past it, especially when the tire flexes under load and heat. It is the most common source of a slow leak that soapy water on the stem does not find. The tell is bubbles at the edge of the rim when the tire is soaped all the way around, or a tire that holds air sitting and loses it running.
The fix is to dismount the tire, clean the bead seat to bare metal, and remount with bead sealer. A wheel that is badly pitted or has a crack at the bead seat is scrap, and it is worth finding that out before it fails under a load. Cracked wheels are also an inspection item, and they are part of what we covered in Loose Wheels.
The tire
Sometimes it is the tire. A nail in the tread, a small puncture that has sealed itself around the object, or a leaking repair. Soap the whole tread and look for bubbles. If the leak is a puncture in the crown, it is a repair candidate. If it is in the shoulder or sidewall, the tire is finished.
Why the inside dual is always the one
Because nobody can reach it. The outside dual gets checked with a gauge. The inside dual gets thumped with a bat, if that. Thumping tells you a tire is flat. It tells you nothing about a tire that is twenty percent low, which is the tire that is quietly overheating. Inflation checks on a dual mean both tires, with a gauge, every time. If that means an extension, the extension needs to be a good one and it needs to be checked for leaks as often as the tire.
What the crew does on site
Find the leak. Stem, core, extension, rim, or tire, with soap and patience. Replace the stem or core, fix or remove the extension, clean and reseal the bead, or repair or replace the tire, whichever it is. Then set both duals to the same pressure and check the tread depth match so the pair is carrying the load together. It is the same tire service call as a blowout, just a lot less expensive and a lot less dramatic. We would rather make this call in your yard on a Tuesday than on the shoulder on a Friday night.
Frequently asked questions
How much pressure difference between duals is too much?
Very little. A small difference in pressure changes the tire diameter enough that the two tires stop sharing the load evenly. Duals should be set to the same pressure and checked together.
Why does the tire leak on the road but not sitting?
Heat and flex. A bead seat leak or a cracked stem opens up when the tire is hot and flexing under load. Soap it right after a run for the best chance of finding it.
Are metal valve stems better than rubber on trucks?
For commercial truck wheels, metal stems are the standard and they hold up better to heat and pressure. The grommet still needs to be in good shape.
Can you find a slow leak on the roadside?
Yes. Soap, a gauge, and a dismount if needed are all done where the truck sits.
Got a dual that will not stay up? Call Duckett Roadside Repair at (850) 495-0366 and get it fixed in the yard before it fixes itself on the highway. Dispatch is answered 24 hours a day.