D4 7U losing water

Been doing a few odd jobs on weekends lately and I've been losing anti-freeze. It goes down right below the core and stops going down. I'll bring it up right above the core and the next time I check the level its right below the top of the core. Doesn't seem to matter if its a 3 hour job or 8 hour job. I have no external leaks. Could it be going out the overflow? Used to not have this problem.

Unless you have foaming, the loss is unlikely to be from the overflow. I would suspect a leak a the tank to core gasket, but it may take a pressure test to find it. I had a tractor that began loosing coolant in a similar fashion about a year ago now. I kept topping it off and even found a leak on a heater hose in there somewhere, but the leak continued. I really expected that I had a cracked head or seeping head gasket because I never could find the leak even though I was using the tractor a lot and topping it up regularly. Finally got a break and lifted the hood so I could see the upper radiator while pressure testing the system and found the leak in about a minute! It was about 1 inch below the top of the core and small enough that it didn't leak much until the engine warmed up. The hot radiator and AC condenser evaporated the coolant before it could even hit the fan to sling it all over.