I bought new rubber stemmed tubes and rim liners in 2017 from Mike's and installed them in 2019 with new Universal 21" tires. The tire pressure remains very constant in all tires, and any changes I think are due to the summer/winter temperature extremes here. If you do want new tubes, maybe contact Erich Lindeman at Mike's and see if they still are using the same tube provider now. I sure like my tubes.