I have to be honest, the idea that the pedal sinks to the floor when you hold pressure on it indicated the fluid is bypassing the plunger in the master cylinder. I'm not sure how the booster would be causing that issue. Maybe you should suggest a different brand master cylinder.
Since it fixes the problem for a couple weeks and then starts again, I feel the MC's they have been getting are bad. Even though one would think an OEM part would be best, they can have bad ones as well. Also, once the system is sealed and working, there is no way air is getting into the system unless you see a leak at some point. Does the brake fluid remain full regardless if the issue?
The master cylinder on this vehicle is a typical design. There really isn't anything different or out of the ordinary. I suspect they have been getting a bad batch of parts. Other than that, check to see if you have one of the rubber brake hoses expanding when the pedal goes to the floor. Have a helper watch them as you press the brake.
Take a look through this link and see if anything is helpful:
Let me know.
Wednesday, January 1st, 2020 AT 5:06 PM