Update: I did buy the TST TM-507SE (external) sensors from Plugitright. I do like the idea of internal sensors but these are working out well. Although Plugitright did not answer my email asking when to expect the package, it actually arrived very fast, faster than I would have even expected to get a reply by email since the order and email were sent on a weekend. That was nice since I had a long trip planned and I had time to get everything set up in time for it.
I did put the metal valve stems on the trailerís tires. The cost was $3 each so it was a no brainer, especially after seeing some reports on the web on how the sensors allegedly caused rubber valve stems to flex (in one case, even to the point of the sensor leaving marks on the wheel when pushed against it at speed). I also opted for balancing, so with tax, etc. it was $27 for the two tires or close to $180 for the TPMS with two sensors and two metal valve stems with the tires balanced with the sensors in place.
I also chose to remove the anti-theft device from the sensors. That made then just a little bit lighter. I donít think the risk of theft is very high and I take them off the wheels anyway when parked for more than a day. That also helps the batteries last longer (they are supposed to only transmit when under pressure).
In order to extend battery
life, the sensors only transmit about once every five minutes unless there is a sudden drop in pressure, in which case the unit will immediately alarm. I canít say how fast or how much the pressure drop must be to trigger an instant alarm but I can say that when I remove the sensors from the valves, and the pressure immediately goes to zero, the unit alarms every time.
In the short time I have owned the TPMS I have put on 1,650 miles and been very happy with added piece of mind that comes from seeing the pressure and temperature on the trailerís tires. Plugitright not only shipped the kit very quickly, they also had a great price.