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