I'm wondering if I should put an overflow tank on my '65. I'm still about a gallon short of what the manual says will go in. When I filled it all the way to the top, I lost quite a bit on the ground when I warmed the engine up. Then when I rechecked after cooling I couldn't even see the coolant in there. Are you not supposed to fill to the top?... I've never had a car w/o an overflow tank.

 

Turn on the heater if it has one on high. If it takes the water and gets warm, then overflows, you may have blockage or a bad thermostat. Is those do not apply, don't know what to tell you. Hows the cap? Correct psi is needed. If it's weak you can get blow by. No reason under normal conditions for it to rise and fall like that do much. I'm not a mechanic but these things I mention are common water issues. Hope it helps or at least make you say, hmmmm.

 

I put an overflow tank on mine. It's hard to get all the air bubbles out of the system. You need to burp it, and the easiest way is to put on the overflow tank and start and stop it after the thermostat opens and keep checking the overflow tank to see if it is at the minimum level. Don't take the cap off the radiator.