Thanks for that Ed. Yes, I had applied grease only to the outer edge of the O rings. I had wondered if the problem is the O rings sticking and taking a high pressure to make them "jump" out. If I can persuade them out of their grooves I will give it a try.

Although "white lithium grease" seems to be the norm in the USA for use on cars, it is not used here in the UK except for highly specialised industrial applications so it is not generally available. I work on the assumption that vaseline (petroleum jelly) will rot or attack the rubber of an O ring but that silicon grease is safe for rubber O rings - is that right?

thanks

Harry