Tire Valve problems of the 2004 Toyota Matrix

I bought a set of 4 "tire minder" brand 28-32 psi tire pressure monitoring valve caps at brookstone on Jul 27, 2004. I installed them on the tires of a new 2004 Toyota Matrix according to the package instructions. (no valve extenstions and making sure "o" ring was in place, pressure measured by 2 guages at 30 psi. ) all the caps held pressure and worked correctly for approx 24hrs. Then the first cap failed while car was parked in driveway releasing the air within seconds. Approx 12 hrs later the 2nd cap failed in similar fashion. Fortunately, the car was not beng driven at the time or else an accident may have recurred. I removed the remaining 2 caps. I contacted brookstone and reported the issue. They offered to refund money but no other action. I still have the caps. I would be willing to send them to a testing lab for analysis.