I had one tire that lost air every 6 months or so. One day I fired some fix-a-flat into the tire and it hasn't leaked since. Keep in mind the tires are fairly thin so the cart rides smoothly. Because of this, the tires are also easily punctured by the tiniest items. On one occasion I found a rose thorn sticking out of the tire. Fortunately, it was in the sidewall and didn't go all the way through. It wouldn't have taken much, though, to penetrate the rubber.

Good Luck,

Gary