back tries leak

ok, the rear tires, both of them, leak after riding on them for a while. the front tires are fine but the back tires leak only when riding the wheeler. they hold air fine until, like i said i ride on them for about 2 mins. the problem occured for both of them at the same time. i have access to a tire changer, i was thinking of breaking the bead then sealing it again. i dont want to use slim unless i absoluetly have to. do you guys think derimming the tire, cleaning the rim then the rimming it again will solve the problem?