[deuce lover]

The copper line from the tank to the hex on the back of the gauge should only have air.The red fluid is only in the hex and sight glass .If your gauge was upside down the red fluid could have spilled back into the line,which I doubt.You should have disconnected the air line at the begining of your project.

If as Robert said that it drained out of the sight glass I suggest you open up that gauge and throughly clean it.