Tim ;
The usual culprit is actually the rust around the bead of the rim where the tire selas , I have some pretty rusty ones come in and mine were not pretty when I dug them out of the sand , I've _NEVER_ met a tire man who'd take the time to properly wire brush off _all_ the rust , I bead blasted mine and they don't leak one iota of air , before you go putting tubes in you'd be wise to dismount the tires and really clean the beads well , this is serious grunt work , needs a sturdy flat place to lay the rim on and a couple good quality wire brushes , maybe some Phosphoric Acid too , the pits can be really deep as long as you get them shiny clean then paint with epoxy paint and allow to fully dry before remounting the tires .
Ay real tire store or maybe a good gas station with shop will have a large tub of water you can dunk the tire into and look for where the bubbles are escaping from , take your time and don't be rushed , mark the leaks with crayon or lumber pencil ~ something waxy that will not come off in water .
I hope this helps...
-Nate
