I don't know if anyone is still interested about the radiator cap but try this site http://www.parts.rvhyd.com/ Good luck on your hydro hot system. The type of coolant used is boiler coolant either pink or light yellow/green. It can be purchased and most rv parts stores. It is advised by all manufacturers that if you drain the system you Replace the Coolant don't try to reuse it. The hydro hot or aqua hot system is very expensive, coolant is cheap.