Loosing antifreeze

Did you replace the rad cap at the same time?
With a direct-drive fan, my slanty will run under any conditions at rad-cap off and not boil the coolant. Perhaps yours did too, with the pinhole leak. Now , running pressurized, it may be forcing fluid thru an internal leak that was pre-existing, just never an issue at zero-psi.
But like said; chocolate milk is the proof.