Master cylinder 4 sure.
I experienced the same situation in another car while on a drive to New York, and before I could get there, I was losing brake pedal pressure on the NJ turnpike! Not a good feeling, so I had to gear down and use the hand brake until I could get to an auto parts store, and then a hotel where I could change it out in the parking lot.
When I removed the master cylinder, I discovered where my fluid had gone....into the brake booster, where the rubber diaphram is, and I knew that I had to clean it completely (if not the brake fluid would destroy the diaphram). So, thinking of what to use in a hotel parking lot at night, I looked in one tool bag, and found my fuel pressure guage....I removed the overflow tube (small clear tube) and hooked it to a vacum on my intake of the engine.
I then placed the tube in the booster, cranked the engine for vacum, and let the engine suck the brake fluid out. Needless to say, the white smoke at the tailpipe was clouding up inthe parking lot, since it was buring in the exhaust. When I was done, I tried to clear the smoke up, but was tired, so I went to sleep, and got up early the next morning, to get back on the NJ turnpike. Everyone was turning their heads while me and my girlfriend traveled on the highway, since we were leaving a thick white smoke trail behind us for miles. My girlfriend asked if it would ever quit smoking.....I laughed and said yes, eventually.
It smoked for almost 20 miles before buring away!
Drivers on the road gave me plenty of space, and they caught up to me after it quit smoking (the ones that tried to motion t me that I was smoking
)
Hope this helps. Good luck.