The seals that are between the reservoir and the aluminum body are usually the culprits. Problem is though, you can't get those things from anybody [that I've found]. Even the m/cyl rebuild kit from GM doesn't contain these rubbers. I ended up replacing my entire m/cyl assembly to rectify the leaks, $240 usd, exch at most any Car Parts.com location...some are even $199 exch.
HTH
Jim