[Tommy]

Some master cylinders have the fluid reservoir mounted directly on top of the MC while others have them mounted remotely and connected to the MC by a length of hose. If the reservoir is mounted directly on your MC, there is a rubber o-ring that keeps it from leaking. If the reservoir is remotely mounted, there is an o-ring on top of the MC where it connects to the hose, and another where the reservoir connects to the hose. .... If your MC went without a leak for a long time before the leak began, I would suspect internal wearing more than an o-ring. If the car has some miles on it and you discover one MC is leaking due to internal wear, I suggest you go ahead and prepare to do the other MC as well. Good luck.