[Mike_The_Grad]

Bingo.
It looks like it's been leaking for a while. Remove the check valve from the vacuum line and look for brake fluid residue inside the rubber line This might give you an idea of how long and how much the MC has been leaking.
If no sign of fluid In the line that's good news, because the MC probably has a small slow leak and may have not been enough to damage the diaphragm inside the booster.
If there is residue in the line, try and shine a flashlight into the brake booster through the hole for the check valve. Hopefully it's not to bad in there, if you notice rust, scale, or any crap inside the booster, then chances are it'll fail sometime in the near future. Don't spray any cleaner or degreaser inside the booster. It'll only damage the rubber diaphragm inside there. If it does fail, it will be very obvious, very quickly. The amount of foot pressure to apply the brakes will increase a lot. You will still have full braking power to the wheels though. Also you will notice the engine running different. When that diaphragm ruptures it creates a pretty big vacuum leak, and your engine will idle and run differently. You may even be able to hear it sucking air continuously.