Radiator cap pressure KB2

My 1947 KB2 has original engine with a replacement Champion aluminum radiator. I had thought that the cap pressure needed to be 4 lbs ( I have switched out the cap that came with the radiator to a 4 lb). I seem to be loosing antifreeze but have not actually seen it come out the overflow and the temp doesn’t go much over 160. No water in the crankcase oil, but I do find a bit of brown water ( feels a little slick like antifreeze) collected in the sparkplug depressions after the truck has sit after a drive. I have checked it immediately after a drive but no loss at that time.The engine runs perfectly otherwise.
I have seen another members comment that he modified his cap to “0” pressure.
I am stuck, any suggestions appreciated.
Thanks

factory radiator needs a zero pressure cap, I'm not sure on Champion replacement ones,

you fill factory rad to about and inch to inch and a half below bottom of fill spout, otherwise it just pukes it out with expansion.

again not sure on Champion replacement, but myself I'd treat it like the original factory one. zero pressure and not overfilled

Thanks, I'll switch to a zero cap. That will be a lot easier fix than all the really bad scenarios that my mind had conjured up.
Merry Christmas