How to tell if a m/c is bad?

How can I tell if a master cylinder is bad? I have a 96 XJ, w/o ABS. I have the stock D30 brakes in front and Explorer discs in the rear. I am going to replace the brake booster due to it leaking around the brake pedal rod. If the engine is off and I put my foot on the brake it builds up vacuum. If I turn the engine on the pedal goes down, but not all the way to the floor. I figure the m/c is still good but just want to make sure my logic is good. Thanks.

-Mike

It sounds as if it is good. Push steadily down on it under power, and it should hold and not sink slowly to the floor. It should also drop to about the same point every time you push it. If it intermittently goes low, that's a not good. When you replace the booster, make sure there is no leakage of brake fluid around the push rod. If it checks out you're probably all right.

The pedal is always at the same level. The brakes do suck though as I almost have to stand on them if someone pulls right in front of me. I expect the reason for this is the vacuum leak in the booster. If the m/c leaks, couldn't I be able to tell that pretty soon after pulling the m/c off the booster? I just hope that there isn't a leak in the m/c, as I know that can contribute to a leaky booster.
Isn't there a special way to tell if the plunger rod is the right length? Something like: if you depress the pedal down only an inch or so the fluid in the m/c should erupt? Thanks.

-Mike