I have a problem with some faces of brushes disappearing. only happens on brushes that have had their verticies messed with. sometimes they will be fine one way, but when rotated they disappear after compile.

anyone have a fix for this, or know how to avoid the problem? thanks.

Invalid solids, if you're messing with vertices make sure that you're not "warping" one of the faces. Try using triangles in place of the brush, because if you check for problems (Alt+P) in Hammer I'm guessing a few Invalid Solids will show up. There is also another possible problem that I have seen time to time where it's just a bad brush that likes to go invisible and your best bet is to just re-build the exact same brush from scratch in place of the old one.