Most likely one or both of your primary piston seals have gone.
Quite common if the master has been laying around a long time. The seals are like a cup and don't take much to wear down or perish. Also, if the cylinder has a steel liner, they can rust from the outside in (brake fluid is hygroscopic (I think that's the word!) meaning it attracts water). This allows fluid to leak past the seal and can also wear out the seal prematurely by rubbing over the rough surface of the rust. Whack a kit through it (if the bore is still ok) and it should be all good. They're very easy to rebuild. Just get a tube of rubber grease and grease the seals well before installation.