I've had a very slow leak in my front tire for a while now. By slow, I mean it takes about a week to go soft enough that we have to stop and get it pumped up. Lately it seems to be leaking a bit faster, so we have to pump it up every 3-4 days. When we try and patch it, we can't find any hole to patch. Could it be the valve? Any other ideas? We actually replaced the tube and found the same problem with the one we put in!