Weird tyre.

In hindsight my fault for not monitoring enough, but noticed it was rather flat when I parked in town the other day, being rather lazy I just plugged in the cheap pump to have enough air to get home. What's odd is this tyre was only flat on the bottom, I removed it at home inspected it, and pumped it up again, and it's still holding pressure.
When I did an oil change on Sunday It looked down so I thought I'd pumped it up but later it appeared the gauge had a fault and maybe all I managed was to remove a critical amount of air, which BTW I do fancy style and withing a budget by using a 80% nitrogen mix, the 100% being beyond my reach.
Somehow though I know this tyre will fail again at a time that will be least convenient to me, so I ordered a couple of new ones yesterday. The spare also needs replacing as it has had nearly 50 birthdays and is really too old to rely upon. At least they're not an expensive size.