I think there are several possible risk factors for valve stem leaks, but I also think that these risks can be mitigated if a little bit of care is taken when the TPMS caps are installed.

1) Metal Valve Stems - as I noted above, one manufacturer (Garmin) recommends that their sensors only be fitted onto metal valve stems. This makes sense, because metal stems are not subject to the same fore-and-aft flexing during acceleration/deceleration (caused by the leverage arising from the weight of the sensor cap on the end of the stem) that rubber stems are subject to.

If you buy good quality metal valve stems from a reputable manufacturer (in other words, not Chinese products off of the internet), and you fit them carefully (cleaning the rim surface on both sides before installing them), you won't have to worry about valve stems for a long time... no need to replace the metal ones every tire change.

2) Make Sure the Schrader Valve is Snug - the Schrader valve screws into the inside of the valve stem. A special tool is needed to snug it up... those of us who are 'of advanced age' remember when metal valve caps on our bicycles had this tool built into the outboard end of the valve cap. Make sure the Schrader valve is snug inside the valve stem - you only have to do this once, when you install the valve stem on the wheel.

3) Buy Good Quality Sensors from a Trustworthy Manufacturer - For example, I consider Garmin to be a 'trustworthy manufacturer' of 'good quality' products. I'm sure there are other equally good manufacturers of valve-cap TPMS sensors. Stay away from no-name Red Chinese products, and be especially aware of the risk of counterfeits.

4) Install the Sensor Snugly, then Leave it Alone - Once you have installed the sensor on the end of the valve stem, refrain from removing it unless you need to add air to the tire. Hopefully you should not need to add air except once each spring and fall - in the fall because of the outside air temperature drop, and in the spring to make up for miniscule leakage during the winter storage period. Don't remove the sensor to check the pressure manually once you have confirmed that the sensor is operating accurately during the first week of use. The tiny O-Ring or rubber sealing surface inside the sensor that mates with the end of the valve stem is a potential source of leakage if it is worn or deformed from multiple compression events.

Michael

I think Garmin is a reseller of the sensor. If you take look a look up at the fcc data base of Garmin products, you`ll find out that Garmin worked with serveral manufacturers in Thailand, Taiwan, Korea and China who all offer wireless tpms products.

I did some technical research these days about wireless tpms connections. I have hacked other wireless devices in the past to adapt them to my needs. I sniffed the data traffic to get information for reprogramming. Did this with an arduino and open source anaylzing software.

This principle is commercialized in the automotive aftermarket and it is called Universal TPMS Sensors. These sensors can be relearned to any car on the market now. Because TPMS isn`t mandatory yet on motorcycles manufacturers just bind serial sensors into their bordcomputer systems. New safety argument and new turnover incl. a new cause of defect(see last sentence of comment!). Garmin acts similar, they offer sensors who send a code to the navi which imho is more market feature to secure a constant retailing price than a security one.

Fun with hacking TPMS devices?
"Some cars don`t let you start the engine when a tire sensor reports a flat today.

So if my neighbor makes angry by starting his loud barking harley (or fat adv bike) early in the morning, I could place my battery powered arduino near to his bike to simulate a flat through cloned data traffic. "

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If you're mainly worried about getting a potential blow-out in a tubed tyre, a TPMS isn't going to help you - other than ping you the second the punctured tube rapidly deflates, which you'll already be in the middle of trying to deal with... while a slower deflation you'll soon feel through the bars/saddle anyway.

The main reason for TPMS is to see if there is a gradual leak in one vs. any of your other tyres, so that you don't continue on a under-inflated tyre and potentially overheat it.

Jx

My 5 year experience with aftermarket TPMS has been such that I consider it essential equipment, it's one of the first farkles I put on my bikes.
I accidentally* bought a 20 buck (including shipping) set from China 5 years ago. When it arrived I was cleaning the bike so I installed it right away. The sensors are those little things that screw over the Schrader valve, the display is about 4 x 4 x 2 cm. First thing I noticed was that my pressure seemed good. And, oh, it has temperature sensing.
'What sort of BS is that?' I thought.
I took a ride, about 50 km, and had forgotten about the display. When I stopped I saw that my tire pressure had increased from 39 to 43 psi in the rear and temperature had gone from about 20° to over 40°C. Front went from 36 to 39 psi and 20° to 35°C.
'That cannot be right.' I thought.
Now I know this to be true and expect these changes when I ride. I used to be one of those people who had to have calibrated tire gauges. Now I know that the pressure differs so much with temperature changes that those gauges only show small part of the whole picture. I haven't even seen my old gauges in years. If I need to measure other tires I just unscrew one of the sensors from my bike, screw it on the valve on the other tire, and check the display on my bike.
Now there is no need to get the calibrated gauge from the protective pouch, unscrew the dirty caps, measure the pressure, put the caps back on, pray that I didn't get dirt into the Schrader valves (that would cause them to leak) and put the gauge back into it's vault, before it is safe to ride. I just check the display, if the pressure is good I ride. The display has alarm, pretty loud, it has gone off as I was replacing my tires.

This system has not been without issues. The display somehow got water inside. And when I opened it to dry it a wire came loose. I soldered the wire back on and put the display where it does not get wet. No problems since then. Every 3-4 months the display shows battery low. Then I take it inside and charge for couple of hours with a USB cable.
The batteries in the sensor went flat after 3 years. Now I replace the batteries in the sensors when I replace the tires.
The display shows the pressure as it was when the bike stopped. It has updated to current pressure and temperature by the time I've rolled the bike to the street.
Issues I have not had is air leaking out of the tires, valves spontaneously combusting from the loads (I didn't even get vibrations from the tires being unbalanced), or expenses from blown tires.
The only expense apart from the system is few button cells.

Unfortunately my TPMS is not made anymore. But China has lot of alternatives, that look near identical, and are even advertised as waterproof.
The latest I've seen is Bluetooth sensors. Same functions but can be paired to your phone for displaying and alarms, which has its own pluses and minuses.

* Don't browse the Internet after few . That can lead to what some call a Dutch bargain, others call it wet bargain.