tyre fitting

Please excuse my ignorance on this topic. I had a new rear fitted yesterday - Pirelli angel st. Did a good 200 odd miles on it and was pleasantly surprised with its performance / grip. I thought id take the bike back home through the Cheddar gorge,just short of the gorge outside Axbridge i flatted, no visible nail / holes etc. Now if my theorys tripe then just say, but are tyres sealed / glued to the rim or literally when blown up they create their own seal? My theory goes that if i were using engine braking too often and i broke this seal through the tyre twisting on the rim it break that seal & therefore dump the air in the tyre?

Don't think so. No the tyres are not glued although sometimes people use a soapy solution to help the tyre slip over the rim. When they do this the tyre has a sort of seal but nothing substantial. You are correct - tubeless tyres create their own seal when inflated.

Regarding the issue of engine breaking slipping the tyre around the rim - this could happen if the tyre was significantly under inflated to start with. So if it was under-inflated your theory could be right however you'd have to ask why under inflated - because it wasn't inflated in the first place or because it had a puncture? Perhaps it just had a leaky valve that somebody had forgotten to check. I'd go for the simple solution first - either a puncture or a leaky valve.

At the risk of stating the obvious, could there have been something under your dust cap, either by accident or messing kids assuming you parked it somewhere this could happen. Could have easily dropped out when you took the cap off.

It might be a case where the original core opened somehow. Your valve core may have worked with 80psi keeping it closed but could be weak with less pressure. If it was my bike, I'd put in a new valve core & run the bike again, checking the pressure every 5 miles (presumably higher once warm) just making sure it stays consistant.

I think your mate has posted the same question on Pistonheads & has had all sorts of answers, but a tyre will either seal or not.