yep, if the system won't pressurise it does not heat up properly, found that recently with a leaky rad. and a year ago with a leaking core plug.
but at the same time i have some image of a physics teacher or college lecturer telling us that coolant is prevented from boiling
because its a sealed system(because of the pressure)...
though my mind does consist mainly of jelly!
the 'engineers' will work it out in the morning i'm sure..
good one! people do always overlook the obvious