Hmm, sounds as if your threads may have gone on the cap, this happened to me in an old Escort.
From what you've said, I don't think it's anything to do with high
pressure, otherwise all the red lights on the dash would have come on.
The plastic will deteriorate, especially if it's on and off a lot, or it's been wrongly threaded at some point
Replacements are easy enough to find from a scrappy or fleabay (about £10).
If the threads look ok on the cap, have a look at the top of the engine, and check the threads on there, might be an expensive garage job, or you may be able to bodge it with some tinfoil, cardboard and a big hose clamp.
Hope this helps