I know this may be a obviously stupid question but I'm trying to pull out the valve from my old booster to put in my new one and I can't seem to get it out, I've pulled, tried screwing it off, even tried to pry it off. Is there a trick to getting it off or am I doing it all wrong?

just pull and twist, its just a rubber grommet holding it in...

Wiggle it around while spraying some silicone spray down the sides...should come out easier then.