There is a at th rear of the Master Cylinder that can leak and Brake Fluid can go inside of the Power Brake Booster.
There is also a outer O-ring on the Master Cylider Flange that seals the Brake Booster off from the atmosphere so when the seal in the back of the Master Cylinder leaks fluid you do not see the leak because it cannot leak to the outside.
See post #6 for a diagram of the Master Cylinder.
http://www.peachparts.com/shopforum/diesel-discussion/359277-83-300td-no-brakes-hissing-vacuum-pump.html