James,
The cap, like other radiator caps, if it is fully tightened has to be pushed down at some point to work by the metal retainer tabs, usually after it is fully loosened, so it should just turn. I usually push down while loosening a cap out of habit but that is usually only needed where the tabs are involved. Try using a wand on some WD40 and flooding the underside of the cap several times, may not help but will not hurt. The cap probably has not been worked in a long time and the rubber has adhered to the brass receiver surface. You have to break the bond and may need a set of channel locks or a strap wrench on the cap, being as gentle as you can breaking it loose. If it seems to require much effort, spray some more and wait. You do not want to damage or break the joint of the brass receiver below. I would try and have a new cap available, some of the old rubber seal surface will likely remain stuck. Some plumbers clear silicone grease/lube, available at HD or Lowe's on the new cap gasket will prevent this issue and aid with the sealing. Good luck, may the force be with you...
Later Ed