Hello Bill. You dont want to fill it with water cuz if the leaking is internal then it will start rusting & must be torn down ASAP but if it is inside & we find it with another method at least we have some time to get after it if we confirm it being on the inside by dry checking. I would get a "tank valve" from O'reillys for less than $3 & with your air chuck & a tire pressure gauge pump the system up to 10 lbs & see if you hear where the leak is coming from & you can squirt some soapy water around the pump & intake manifold and head/block interfaces to see if it bubbles. it is a schrader valve with a 1/8"PT fitting & screw it into the cooling temp sender port on the water pump. EDIT pic added. MORE EDIT if it wont hold vac it wont hold psi as it is a simple sealed system ex for the possible (not sure) complication of the type of rad cap with the loose thin brass "recovery" piece dangling on the bottom. with a "regular" cap if it wont hold vac then there is a problem. I had a weak cap so I put a piece of rubber under it to seal the system when I was dealing with my issue.