I know you said it's not the valve, but.... On mine the valve stem developed a crack where it passed through the rim. The valve itself wasn't leaking, but the one tire kept going flat with no visible leak. It wasn't until one of the times I was pumping the tire back up that I deflected the stem a bit and heard air rushing out. The valve looked perfectly good - the crack was hidden from view. New stem, no problem - something to look for anyway,