Old tool restoration - wooden spirit level - remove old red caulk


Really easy to find decent old 2-3' wooden spirit levels that need jut a little TLC to spiff them up and they are often constructed of really nice wood.  Typically these are available for under $5 or so at flea markets but often one or more of the glass "windows" protecting an intact spirit bubble needs to be replaced and this glass window is encased in "red caulk" that appears to be about as hard as cement!  Anybody have a decent method or suggestions to remove/replace the old red caulk?  Surely the old timers must have been able to repair replace a busted glass window?

I realize that there is probably not a lot of demand for these old wooden spirit levels but they do make for decent eye candy when they are cleaned and restored and at the least I have used pairs of them as winding sticks.