A faulty tyre valve can leak air just like a puncture, and it's a common cause of a tyre that keeps going soft. Rubber valves perish and crack with age, and the small core inside can fail too. The fix is cheap, and the golden rule is to fit a fresh valve with every new tyre so it lasts as long as the tyre does.
How a valve leaks
The valve is the small stem you inflate the tyre through. Air can escape in two places: around the rubber base where it seals to the wheel (especially if the rubber has perished), or through the tiny spring-loaded valve core (a Schrader core) inside. Both cause a slow loss that mimics a slow puncture, which is why a leak isn't always the tyre itself. Ozone, road salt and years of flexing are what harden and crack the rubber stem, so a valve on a car that lives outdoors ages faster than one in a garage.
Replace the valve with the tyre
A rubber valve has a limited life, much like the tyre. Because the valve is exposed when a new tyre is fitted, the sensible, standard practice is to replace it at the same time, for a tiny extra cost, so you don't fit a fresh tyre onto an ageing valve that may leak in a year or two. A new valve with every tyre is cheap insurance against a future slow leak.
TPMS sensor valves
Cars with direct TPMS have a pressure sensor built into the valve, which is more expensive than a plain rubber valve and has a battery that eventually dies. These need careful handling during a tyre change and sometimes a service kit or replacement. Valve makers such as Schrader supply TPMS service kits (a new grommet seal, nut, valve core and cap) that are designed to be renewed each time the tyre is removed (source). If your car has direct TPMS, it's worth knowing the valves are part of the sensor, not just a plain stem.
Finding a valve leak
A leak from the valve is easy to confirm with the tyre off the wheel, using a water bath or leak spray to see exactly where the air escapes. That's the only reliable way to tell a valve leak from a tread puncture or a rim leak, which is why a proper inspection beats guessing.
Signs it is the valve, not the tyre
A valve leak usually shows up as a slow, steady loss of a few psi a week on one tyre, with nothing to find in the tread. Punctures tend to be less tidy: the loss rate changes with speed and load, and there is often a nail or a scuff to see. These pointers make the valve the prime suspect:
- The cap has been missing for a while. Grit and water get into the core and stop it seating.
- The rubber stem looks cracked, chalky or bent when you push it sideways, or it is stiff rather than flexible.
- You hear a faint hiss with your ear near the stem, or feel air on a wet fingertip held over the open valve.
- The same tyre keeps dropping after a puncture repair that was done properly.
- The car has done several years on its original valves and no one has replaced them with the tyres.
- The loss is worse in cold weather. Cold stiffens perished rubber and opens small cracks, on top of the normal pressure drop explained in tyre pressure in cold weather.
None of these is proof on its own, but two or three together mean it is worth testing the valve before booking a puncture repair.
The soapy water test at home
You can check the valve itself without removing the tyre: a little soapy water on the stem shows a leak as a slowly growing bubble. It takes five minutes and needs nothing more than washing-up liquid.
- Inflate the tyre to the correct pressure and remove the valve cap.
- Mix a few drops of washing-up liquid in a cup of water and brush or drip it over the open end of the valve and around the base of the stem where it meets the wheel.
- Watch for 30 seconds. A bubble forming at the open end means the core is leaking. Bubbles at the base mean the stem seal has failed.
- If the core is leaking, a valve core tool lets you nip it up a quarter turn. If it still bubbles, the core needs replacing, which costs pennies and takes a minute.
- No bubbles at all points away from the valve and towards the tread or the rim seal.
A leaking stem cannot be fixed with the tyre on the wheel, because the rubber base sits inside the rim. That is a job for a fitter with the tyre broken off the bead, and it is worth doing at the same time as any puncture repair, since the tyre is already off.
Which type of valve your car has
Most cars without direct TPMS use a snap-in rubber valve, while cars with direct TPMS (a sensor in each wheel) have a sensor valve that needs more care at a tyre change. Tyre valve types explained covers each type and how to tell them apart.
Caps, cores and the small parts that matter
The valve cap is not decoration: it keeps grit and water out of the core and acts as a secondary seal. Replace a missing one straight away. A few other small details save a lot of slow-puncture callouts.
- Use plastic caps on TPMS valves. Aluminium sensor stems and cheap metal caps corrode together and can seize, and freeing a seized cap has broken many sensors.
- Do not overtighten a core. A quarter turn past snug is enough; more damages the seat and makes the leak worse.
- Ask for new valves with new tyres and check the invoice shows them. It is standard practice, but worth confirming.
- Do not use tyre sealant to fix a valve leak. It does not reach the problem and it clogs cores and TPMS sensors.
The law cares about the result rather than the cause: a tyre that is under-inflated because of a leaking valve is still a tyre that is not fit for use, and the requirements are set out on GOV.UK.
On cars with TPMS, a persistent low tyre also lights the dashboard warning, which is an MOT failure if the system is faulty.
We handle valves
If a tyre keeps going soft, we can check whether the valve, the tread or the rim seal is leaking and replace a perished valve, at your home or work across London, the Home Counties and the Midlands. Book a check.
