Usually this is from using inner tubes with a fair amount of real rubber.? Or, so the theory goes. Occasionally it is due to problems with sealing at the stem caps¡.or¡occasionally a tube is actually punctured, possible has a sealant inside that doesn't do what you might have thought. I assume you are using tubes and that you check the pressure carefully, and always when the tire is cool (not heated from the sun or riding). How much pressure do you lose over, say, a week's time?
