Ok I think its time for basic lesson in how the cooling system works and what exactly it does.
We need water and coolant in our cars year round (usually a 50/50 mix depending on type of antifreez) the antifreez protects against freezing in the winter, protects the internals of the engine and rad against corrosion and keeps the boiling point at a managable level all year round, Too much antifreez and it wont cool proper and not enough it wont do its job.
As the coolant gets hot it expands and this is why there is a need for a pressurized system so it doesnt just evaporate on us.
there are 2 types of radiator caps/overflow systems that i know of, the old school type that merily dumps fluid onto to the ground, which i belive you have and the newer cap that blows it to a tank and when it cools down sucks it back in.
with the old school type cap your level of coolant when cold will be lower then the top of the rad, 1 inch 2 inchs maybe more it just depends on the car.
with the newer style the level should be at the very top of the cap or right near it when cold.
I say leave the 13ib cap on there, make sure you dont have a cooling system leak (pressure test or merely drive it) put in the proper mixture of coolant fill it right to the top, go for a good long drive heat it up fulley, let it blow out what it wants to. then let it cool and go for another drive and let it cool do that oh say 3 times. Ill bet by the 3rd drive it wont be blowing out any more coolant
and when you check the fluid level (when cold) its down a couple inches
(which is fine)
youve got a good temp gauge that is good
-gbm-