Cadillac Repair: fading brake pedal, cadillac deville, master cylinder

Question
We have a 93 Cadillac DeVille. The brake pedal felt spongy and bleeds down at a long stop and lets go. We had a master cylinder put in about 6 months ago with the same problem. Now we developed the same problem again so they said that we should put in another master cylinder(they are both are rebuilds). Still having the same problem with the pedal. We are at witts end. We would appreciate any information or opinions you may have. Could there be something else going on? Please help.

Answer
If your NOT losing any brake fluid from the master cylinder then you most likely have a faulty master cylinder.

QUESTION: with the engine running and the brake applied can you hear a slight hissing noise down near the brake pedal ?. If so, The BRAKE BOOSTER is leaking and requires replacement.