I've never seen a bellows go "porous". If anything the bellows will leak in a few places, e.g. at the corners where it's worn or where leather inner corners have dried up and cracked. You should be able to detect those leaks by hearing or feeling the air blowing out when you compress the bellows. Patching it with very thin leather might help. A bookbinder's supplier might be a good place to go for that (that's a tip from a friend who was repairing a concertina from scratch. N.B. concertina bellows are different from melodeon bellows, but we're talking spot repairs here).
Leakage is otherwise the sum of dozens of small leaks from worn and/or misaligned pads, and fixing them is hard work: basically replacing every one if you want to be sure.
If the instrument is not too old the bellows may be replaceable in its entirety, or a replacement can be manufactured as the bellows as a unit is easily separated from the rets of the instrument.