When you need to replace your master cylinder

I was told by a brake specialist that my master cylinder was leaking on the backside. How do you know when it is time to replace it?

That does not sound like a very specific technical explanation if that is what they said. When mine failed, the brake pedal sank slowly to the floor. I have heard of others that sank immediately to the floor and required a second push to get some braking. If your pedal does not sink to the floor and if the system does not lose brake fluid, then ask some questions. What is the back side? I don’t know.

Why was he looking at the brakes? Wake mode and year of car? Past history?