Do they work? Every couple of weeks my husband checks my kids tires, it would save him some work.
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They work but they can introduce problems ,, those gadgets keep the schrader valve (in the valvestem) open a bit to monitor the pressure, these gadgets are made of a hard clear plastic that can crack if you just bump a curb or you could simply have a leak where it screws onto the stem... additionally you have to remove and re-install them every time you add air,, their seal to the stem will deteriorate...
Here are two systems that can monitor tire presure in realtime ... very soon monitoring systems will be standard equipment (mandated by US-DOT/NHTSA)
http://www.tirerack.com/accessories/detail.jsp?make=Orange+Electronic&partnum=P409S.TPMS&ID=55
http://www.tirerack.com/accessories/detail.jsp?make=Hella&partnum=HELLATPMS&ID=51
They have a one-star rating at Amazon. Lots of failures cited.