The fluid will wind up in the booster, and eventually it will damage the diaphragm, as well as fill the booster with fluid, which will reduce the effectivness of the booster, meaning the pedal will get harder to push...
There is a seal between the master and the booster, but most of those by now are really nothing more than rubber dust...
Unbolt the master cyl, leaving the lines connected, and pull it forward, away from the booster. If it is leaking , it will be obvious.

Hey Joel,

Well...iffin it were mine, I would most likely rebuild the master cyl meself, but I realize that's not an option for everyone. In that case, then you can buy a rebuilt master and install it.
The seal I mentioned above is not part of the master cylinder...it is part of the brake booster. It seals the rod on the booster that actuates the master. The outer part of the same seal also seals against the master cyl where it bolts to the booster, so, theoretically, there will be a tight seal so no vacuum leak occurs. However, if the booster seal is bad, AND the master is leaking, then the leaking fluid will go into the booster, since there will be vacuum pulling it in there. If the master is NOT leaking, and the booster seal goes bad, then it can pull fluid past the master cyl. seals, and create a leak there.

I had a vacuum leak on my '74 that I had a hard time finding. It took me several years to finally locate it(I don't drive the car much, so it wasn't exactly a priority). I finally discovered the seal on the booster was the culprit. The seal is not available anywhere, at least not that I have found. I wound up using a seal for the headlight actuator...worked poifectly!
If your master itself is leaking, it's time to either replace, or rebuild it, depending on you mechanical skills.
Adams' Apple2009-10-25 07:54:07

We'll go with a new Master Cylinder.

Kevin, if you get new master cylinder, make sure you bench bleed it before installation.   

Also, when replacing the master, be sure to check the depth of the piston hole where the push-rod goes into the master. There are two different depth piston holes for different year models. One is very shallow, and the other is quite deep. If you put a deep hole master on where a shallow one was, you won't have any brakes, since the push-rod won't be long enough to push the piston in the master.
If you get a shallow hole master, and try to put it on where a deep hole master was, it won't bolt all the way on without bottoming the piston in the master...in that case, you won't be able to push the brake pedal down.
I think most of the parts vendors have the dimensions listed...lemmie look and see can I get a pic to show what I mean...

Looks like the '77-'82 cars had the "shallow" piston masters. I remember one time I had rebuilt a master off of a friends '81 after I had replaced his with a reman. I kept the original one. Then, one day, the one on my '74 took a dump. I figured, "Hey...I got this one here that is rebuilt, so I'll just slap it on and go!"....NOT! Mine took the longer push-rod/deeper piston, and the later model was the short rod/shallow hole master. Sooooo....I had to rebuild mine...
Adams' Apple2009-10-25 08:20:29

Bench Bleed --- Check

Well, it's done!

Was the old master actually leaking, tho?

Yeah, it was a goner.