I checked my TM tires and three of the five (dual axles) were low/about 20psi. All five valve stems were cracking and/or splitting where the stem comes out of the wheel. Two tires lost air when I wiggled the valve stem prior to checking the tire pressure. They were also cracking around the base of the stems; they would not hold air.
Good thing I checked several days prior to taking off, rather than the customary night before, as it took me a good part of a day to remove the tires, take them to an auto shop (two trips) have the stems replaced, then reinstall the tires.
I was told the original stems were "low quality", and most probably from offshore manufacturers. Such stems are made of inferior materials that don't stand up well over time to the sun, ozone, heat and cold temperatures. I learned that there is a huge difference in the long-term reliability of these cheaper no name valve stems.
When replacing valve stems ask if they meet the SAE 1205-1206 standards. Off shore products usually are no-name products. Stems meeting USA standards always have their manufacturer's logo or country of origin on the bottom of the stem (which cannot be seen when a stem is installed). When having stems replaced ask to see them before installed. If no logo or other identifying marks are on them, be very suspicious. Reputable manufacturers' valve stems are QS9000 or ISO9000 certified, which means they meet stringent quality control standards, procedures and recordkeeping.
So, words to the wise--often check your tire pressure as well as the condition of the valve stems. If the stems show signs of deterioration, cracking being hard or brittle, and the like, have the valves replaced. Taking tires off a TM is no fun, but it is down right dangerous if it has to be done on the road.