1999 k3500 brake problem

i have a 99 k3500 4x4 dually. my brake pedal goes almost all the way to the floor but the brakes still work for the most part.the brake fluid is also disappearing. is it the master cylinder or brake booster or both.

It could be a master cylinder leak, and could just as easily be a steel line, caliper, hose, or ABS unit leaking. Only an inspection of the system components will reveal the cause.