My first guess is the check valve. There’s been a lot of similar stories and most times it’s been the valve. The leak you noticed on the 444c is a good indicator. They have a habit of gumming up so either they won’t let air through or they’ll leak. Either way, try swapping that out before buying a new compressor or anything.
Once in a while, it doesnt make pressure (gets stuck at say 80, or 110 etc), and just runs hot. If I let it cool down, say for 20 mins, and turn it back on, it tops up quickly. Very odd, and much different then my 444
I do have some moisture, and freezing is definitly an issue, but I dont think its the PS. Due to the fact that the compressor will run for 10 mins straight, and be stuck at 80PSI/100PSI whatever, if it was the PS, it would be on/off.
I would definitely drain your tank on a regular basis due to being in a freeze zone. It does sound as though the check valve could have been too tight or bad all together. I would also suggest to never run the compressor while the vehicle is OFF.