Quote Originally Posted by phil2580 View Post
No, my original statement was correct.

The cap I had fitted when I was having problems was not releasing pressure when it should. It was too tight-fitting in the neck of the expansion tank, so would only let the excess pressure out when it was already way too high. All of my hoses were rock-solid when the cap was in place, so there was clearly too much pressure in the system.
The cap should release pressure only when you have an over-heating problem or exhaust gasses in the coolant. The blocked cap was telling you something was wrong, did you check what I said in my other post re pressure from a cold system?