had a slow leak on one of my Swampers. Problem was is that when I "air down" to 12-15 psi to go wheeling, the bead is vunerable to obstructions such as dirt, sticks, weeds, mud...etc
These cause it not to seal properly and after you inflate it leaks slowly....mine started out to go flat after one month...then one week...then everyday...then overnight.
Went down to tire shop and had them remove the tire from the rim, clean the bead area, and then put sealent all around the rim where tire sits and then remount and balance. Problem solved!!!