So your car has absolutly no drips? If you have to add fluid, you have a leak.. period. If not leaking outside the car, the leaking inside is the next place to look. Check the carpet on the drivers side looking for a oily feel.
Hmmm, I think the 93 has the standard vacuum boost master cylinder, so you could also blown a seal in the vacuum booster and the fluid is filling that up. Have you noticed a reduction in assist power?
The loss of stoping power is coming directly from the fact that fluid is leaking out, thus a hard pedal just isn't possible, which means full force at the brakes isn't possible.
If you are certain the leak is not coming from one of the wheels or a brake line, it has to be leaking inside the master cylinder.