I put tire repair goop in my tires. The first time I did it was maybe 10-12 years ago. Last year they started leaking again, and I put in some more. I think I've added air once since then. I used Slime brand this time. It claims to seal holes 1/4" in diameter. We put some in one of our club's trailer tires that I thought it would never seal. One large ball of green slime flew out of the tire, then it was sealed.

http://www.slime.com/

John