Quote Originally Posted by Chernobyl View Post
This is exactly what it's like. I can 'pump' the brakes to get it to stop about a third of the way down, but once I'm stopped, it slowly sinks to the floor, and I doubt it's a leak, since fluid levels have remained high, and I've never noticed any leakage in my garage.
The master cylinder can leak internally.

You don't lose fluid - the internal rings are worn and it's allowing the piston to push past what should be a sealed interface within the master cylinder itself.

There is some good information here.