think of it this way when you make a map... if you where to make a real model of the map and throw it in to like a bucket its supposed to keep air inside, everything in a level has to be inclosed, if it isnt things get a little screwy. leaks happen when two walls dont exactly meet in a way that the level will be closed off, prehaps you didnt make a wall high enough or long enough, maybe you forgot to finish off part of a room. to fix this... after you figure out where the problem is... fix the problem by extending a brush over the hole so that it makes contact with the other brushes and thus plugs the hole... or just make a brush the exact size as the hole... brush entities do not fix leaks so putting doors or things like that wont work