We find we need to add a little air every few weeks, infact, I need to do it
tonight.

Couple things to check. How old are your tubes? You may need new tubes.
Tubes are usually natural rubber and will age and crack.

Do you have steel stem caps that you can tighten with a wrench? The steel
stem caps will have a rubber seal that seals against the rim of the valve
stem. Just don't over torque and rip the base of the stem.

Jim