A 15 lb. pressure cap will raise the boiling point to 257 degrees with plain water, slightly higher with a 50/50 antifreeze/water mixture (Not 18 degrees!) The only purpose a reservoir tank serves is to give the occasional puke somewhere to go to be recovered once the temperature goes down and a suction develops in the radiator. The puke can hose should always be submerged in the coolant in the reservoir so it can siphon the coolant back into the radiator on cool-down. It sounds like your radiator isn't doing its job; either it's too small, or dirty, or both. A fan shroud will also help to keep the air flowing through the radiator instead of around, or behind it. Position the shroud where the fan fits halfway into the opening for best efficiency- - - -the end of the shroud should cover only half the depth of the fan blades; it should not have the blade fully enclosed, nor stop ahead of the blade.
Jerry
