I believe your issue is hopefully less complicated than mine. You will have air in the system as a result of your master cylinder running out of fluid. You will need to fully bled the system when you replace your master cylinder. This will do two things for you: Remove the air in the system and flush out all the old brown fluid. When you remove that old master cylinder check to see if fluid was exiting it at the rear seal and going into your power brake booster. If it is dry there, then you have a leak somewhere else you will need to find.I just got the sinking pedal yesterday, talked to my stepdad who used to be a mechanic, then read through everything on here.
I found the front reservoir to be out if fluid today, and rear reservoir was definitely low. Just pouring in more fluid, I was able to get more resistance in the pedal with the engine on, but also with reduced braking. At idle, I had to jam the pedal down hard to keep it from moving, and the pedal would still sink to the floor. With the engine off, I can pump the pedal, and build some pressure, but it isn't going hard like I would expect. I didn't see any fluid on the ground, or wheels. I did open the reservoir again, and the fluid looks brown in there now. My suspicion is the master cylinder has a bad seal somewhere, but it isn't at the booster, because I pulled it off the booster and had no fluid there. Brake pedal still sinks to the floor, and I have a new master cylinder that I ordered tonight before reading through all this.