What are the chances of having a defective valve stem or getting a flat while driving because the rubber valve stem has a crack?

 

I've seen them leak or the core can get loose and leak too. I've got metal ones on my car that got loose and started to leak a little. After I tightened them they have been fine for years though.

 

Are the valve stems old? Do you park real close to curbs/have you had a near miss with a curb or deep pot hole @ speed? If no, then valve stem defects are not likely.

 

I work in a tire center, and see them come in often. Id say most cases are from that tire shine stuff that dries out and cracks your tires as well as valve stems. You should be fine though if you don't use that stuff, be sure you get new vavle stems everytime you get a new set of tires as well cuz a lot of shops are cheap like that, leaving the old ones in. I have steel valve stems on my car as well and have never had any problems, just be sure to have someone install them that knows what they are doing cuz like peaty said they will leak on ya if not properly installed.