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Need to borrow one of those Stant radiator pump kits & see if the cap holds ~15 psi b 4 it releases and pull into the driveway w it hot/shut it off/take off the cap and pump the rad up to 15 psi & let it sit & check back and on a small leak may need to let it sit up overnight but w your amt of loss it prob wont take that long for the gauge to drop (hope it's external so you can spot it) EDIT other options a rad shop can draw the air in the rad neck up into some blue fluid w their kit & it'll turn yellow if combustion gasses are present and if you have an emissions testing stn closeby they can set the probe in the same airspace in the rad neck & know in seconds if there are combustion gasses present


NEVER EVER REMOVE A RADIATOR CAP ON HOT ENGINE UNLESS YOU WANT TO GET BADLY BURNED. COOLANT WILL BLOW OUT AROUND THE CAP AND POSSIBLY IN YOUR FACE. Anyone who knows anything about cooling systems will tell you Not to do that.