Last Saturday, while doing a routine vehicle check at an event, we discovered that all 4 valve stems were severely cracked around the bottom of each valve stem near the rim hole.
I recommended the owner replace them as soon as possible and I recommend you check you valve stems for cracks. Replace them if there are cracks. You risk a rollover if the valve lets go at highway speeds.
30 million defective valve stems made in China were installed in tires in the United States from Late September 2006 thru mid 2007. They forgot to add the UV protection or possibly ozone protection and the rubber is breaking down. The valves were manufactured for Dill Air Control Products of Oxford, N.C. by Topseal Shanghai Auto-Parts Co. Ltd. in China.
To be sure if you have one of the bad valves you need to dismount the tire and check the manufactures code on the valve from the inside. The bad codes are Dill TR-413CH, Dill TR-414, Dill TR-415, Dill TR-418 and Dill TR-423. But you can tell if it is cracking by bending the stem and looking for small cracks near the rim hole. See the picture I took of a bad one. It is worth replacing a cracking valve stem even if it turns out not to be one of the recalled valve stems.
I don’t know how long it takes for the rubber to break down. It is possible the problem may just be showing up now. The National Transportation Safety Board only opened their investigation last year in May 2008.
Don’t take a chance inspect your valve stems!