I found this info useful. IMHO, plaster is a little bit easier to replace I think - hard to scrape broken glass and cured RTV out of a small hole if the vial breaks again. BTW, in a nonproduction environment you can do what is called "bucking the level". Get a surface that's pretty close to level. Don't bother leveling it unless that's really easy. But you can test a new vial in wet plaster using the surface. Note how far out of level it indicates. Then reverse the level and compare. Adjust to cut the difference in half. Switch the level back and check. For the level in a combisquare, though, it may just be easier to level your surface plate with wedges or some such (bucking is often done with the ultraprecision levels).