My TPMS light came on a few days ago, and I checked the air... both rear tires are down to 31psi. I filled both up to 38psi with the included pump and reset the indicator. A day later they're down to 31. I have a leak, right? If I do, it should go down farther... so I left it another two days to see which one's really got the leak. Both are still at 31psi. Do tires ever have leaks at only higher psi and come down to stabilize? Maybe the pump didn't actually add air and the valve stem kept the pressure for the post-reading only?