Valve cores, caps and slow leaks

The valve core is a tiny spring. Dirt or a half-turn from a sloppy compressor will hiss for weeks at a rate you blame on weather.

Caps keep water and grit out. Metal caps on alloy wheels can seize; plastic caps with a rubber seal are enough for most cars. TPMS stems often want the cap the maker supplied.

A bubble of soapy water at the valve, the bead, and the puncture finds leaks faster than another 200 miles of guessing.

Corroded rims, especially on winter steel wheels stored damp, leak at the bead. Pressure checks every week that always need 3 psi on the same corner are a workshop job.

When you replace tyres, ask for new valves. A ten-year-old snap-in stem on a new tyre is a leak scheduled for next spring.