Replacement cap bad? Find a short piece of 3/8" rubber hose, place it on the vent hole in the cap and try blowing through it, you should be able to release the check valve easily. If not cap is still bad.
Something to check. The plastic fitting that pushlock onto the heater coreabove the downpipe can drip into the exhaust and never hit the ground. I would go rent a pressure tester from an auto parts if you don't have one and see if you can get it to drip out any where
It could also be pooling in the valley and evaporating. Ive seen one that was deleted doing this bc the owner uesd a hose clamp instead of a spring clamp on the cooler by pass fitting and it leaking out slowly only when the truck was hot which it then evaporated. We found it by pressurizing the system with dye...