Dear Tom and Ray:

I have a 2000 Toyota Sienna van. My van tires lose air on a regular basis. The tire-trouble indicator on the dash is on most of the time. The front desk personnel -- who have had some mechanical training -- say I only need to have the tire rims cleaned and sealed, and they should hold for two years. One of the mechanics, who seems to be knowledgeable, says it's because my rims are aluminum, and aluminum rims collect residue that gets between the tire and the rim. He recommended getting stainless-steel rims. Who is right?

-- Jean-Marie

TOM: Well, there are millions of cars with aluminum-alloy rims out there, and their tires are all holding air. So I don't think it's the type of rims you have, Jean-Marie.

RAY: There are two things I'd look at. One is your valve stems. The valve stem is the black rubber thing that sticks out of the wheel. It's where you put the air when you fill up the tires. Valve stems can sometimes crack, or deteriorate and get porous, and allow air to slowly leak out.

TOM: In fact, some 30 million Chinese-made valve stems were recently recalled because they may fail. They were manufactured for a company named Dill and distributed by Tech International all over the country. If you have those, you can get them replaced for free.

RAY: The other possibility is what the front desk personnel suggested -- that you have oxidation on the inside of the rims, and that's preventing the bead of the tire from sealing tightly against the rims. That's also a common source of slow leaks.

TOM: So, my suggestion would be to have your mechanic remove all of the tires, clean up the rims, replace the valve stems and remount the tires. I'll bet that fixes it.

RAY: And if he happens to notice any huge nail holes while he's at it, have him patch those up, too. Good luck, Jean-Marie.