Coolant issue

[Brock]

I'm having a brain fart here... on my 08 Cat c15 when it's running, the coolant level goes down.

When your coolant gets hot, doesn't the level usually rise a bit in the expansion tank? Mine goes down 2.5" like it's being pressured from the top. When I shut it off and relieve the pressure, I can hear a crap load of bubbling from ansmall hose that runs from the top of my resevoir to the thermostat housing.

I did just have a coolant pressuring issue where it was over spilling to the other side of the expansion tank but that was a bad compressor and there's no compression gasses now and it's not over filling.

Im thinking something stupid like a air bubble. Could a bad thermostat have anything to do with this or do you have any other ideas?

I changed the pressure cap with a new one. Maybe there's a internal check valve that's bad in the tank?

Thanks guys.