Often the vials were set with plaster of Paris, at least the glass ones. To replace them you would chip out the old PoP and clean it up good. Now, find a second level that you trust. Find or creat a level area. Mix up you PoP and get ready. Place the level to be repaired on the level surface. Slop in the PoP and insert the replacement vial. Work quickly, make sure the replacement vial reads true. Let the plaster cure. Remove whatever needs to be removed to replace the cover glass.

If it is the plumb vials much easier. Use a plumb bob to find the true.

Kevin

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