After 3 days of sitting I noticed the front tire on my VFR was completely flat. I refilled it and spent about 15 minutes with a flashlight looking for a puncture. No pinholes, no marks, nothing. That was about a week ago. Since I refilled the tire it hasn't lost a single psi. I even rode it about 40 miles to see if that would cause it to lose pressure. What would cause that? I'm thinking there could have been some crud in the valve stem that kept it from completely sealing. I check pressure before pretty much every ride, so I'd know if there was a slow leak over time. There wasn't.