While it is recommended that "if you carry more than 80 pounds of air in your tires or if your tires are 3 years old or older" you change to metal valve stems but it is not mandatory.
Rubber stems over the years, especially if you travel in hot dry weather will start to crack over the years and lose their resiliency. That is where the weight of any sensor can cause the stem to bend which can cause a slow leak.
We traveled over 300+ miles with rubber valve stems and the only sensor to alarm was the one that was 3 years old. We pulled over and examined all the tires and only the one that was 3 years old had a stem that was dry.
That said, we did decide to change all 6 of our 5th wheel tires to metal valve stems. Hugs, Di
slides wrote:
Is it absolutely a fact that you have to have metal valve stems in order to use tire monitoring systems