CO2 leak at ball valve connection

So, I have a leak and drained a tank in about a weeks time. Before hooking everything up did a leak test by submerging the splitter and connectors in water finding a small leak where an MFL washer was left out. Then charged the system and turned off the tank and the gauges did not move for two days, hooked it to the keg and sprayed everything with starsan and a bubble solution for good measure and no leak.

Funny thing is when I connect my setup with a 20oz paintball tank it does not lose any pressure, guess my question is would it be possible to have a bad connector that only leaks when hooked to the keg? Can't think of anything else that it could be, thanks in advance for any help.

For me my seals were bad on my Cornelius Keg. I'm not sure if you're using new or used kegs? But once I replaced the O rings and my CO2 Purge Spring, all was right in the world. If everything seems good with the Gage I'd check the keg. If the keg's good, i'd then check the connector and your right it could be the CO2 connector. They only cost about $4 to replace if it's warn out....

did you crack the tank valve to blow any dust out before you attached the regulator so it seals good?

When you say ball valve, are you using a plumbing ball valve to hold gas somewhere in the system?

Did crack the valve before putting on the gauge, it holds pressure in the tubing and regulator because the needles did not move for two days after turning the tank on then off.

Meant to say ball lock disconnects, thinking that the one on the 5lb tank is bad when hooked up to the keg. Like I said, the small tank did not lose any pressure after hooking up and charging the system then turning off the valve. Gonna have to do another test in a few days when I get time to get a fill, will change out the ball lock connector on that line and see what happens.