Inside Rear Tires Leaking Air Any Thoughts?

[Rich Franklin]

I bought new Michelin's (DOT 2012) last year and for some reason I keep losing a considerable amount of air (25-50 psi per week when sitting) from my inside rear tires. I suspect that it may be because the valve extenders (rubber) they installed however they were all that was available at the time. I've made arrangements to have them inspected and replaced if necessary but was wondering if there could be anything else. I think it's pretty unlikely that both inside tires could've developed a leak simultaneously from a nail, etc., but stranger things have happened. Any thoughts?

Rich