| Could be all of the above-- usually the bridge is centered on (or close to) the inside "nicks" in the f-holes. It sounds as though you have it too high...
When you say it is breaking "at the top", do you mean at the tuning machine string-holes, or at the nut (the crosspiece at the top of the fingerboard)? Either way, you may need to take a tiny round file, and smooth the edge where the string is being cut.
You can lubricate the string groove (in both the nut and the bridge) with graphite from a regular #2 pencil...that will minimize drag as the string is coming up to pitch. Seems to help, anyway.
Even a cheap bass, set up correctly, should not keep breaking strings. What kind of strings, anyway? Any chance the strings are the problem? | 
