Nipper you're the man! You've go tsome great posts!
So where the cap seats is PLASTIC and it looks like its been eaten away. There is a lot of scale in the reservoir. The cap looks new but i don't think its seating right. I might take an xacto knife and try and clean it up a little.
The radiator needs to be able to pressurize to about 30 psi to work correctly, right? Do you think that's what's going on? Am I just boiling over cause of the bad seal around the cap?
Thanks for the reassurance on the HG, phew! :-p