yeah, don't add coolant sa radiator kasi mag-overflow lang yun sa reservoir pag warm up ng engine. kung low ang level sa reservoir add ka lang sa reservoir up to full mark.

or alternatively (not recommended) add sa radiator (with engine cold) pero don't add anymore sa reservoir. problem with this is you don't know how much you are adding so pag sobra maging over naman sa full yung reservoir.

normally you add a 50/50 mixture of distilled water and engine coolant like ethylene glycol, nalcool inhibitor, etc. pag green glycol yun, pag red nalcool.