I dont have a booster, but i thought that the MC was the same, which would mean that it couldnt get fluid through it??? Unless its leaking through the back of the MC i guess.
http://auto.howstuffworks.com/power-brake1.htm Shows how they work. I had the same problem with my GEN light, check your voltage regulator. Check out my old thread on that!
http://forums.mustangworks.com/showthread.php?t=48538