1998 Ford Taurus Master Cylender replacment.4th time! In

Hey guys. I have a big concern here. A while back. My brake likes eroded and I lost my braking ability. So I had the lines replaced along with the master cylender as it was shot.
i har a referb put in, 2 weeks later. My peddle was on the floor and no braking was available!
here and there the ODD time it would feel normal again, but then the pressure dies and I cant break. So i've upgraded the part to a better quality cylender, and the same issue has come up again!
im very frustrated. The resivoir is always full. I've had complete service done to the fronts and Rear Drums. My rear brake cylender for the drums are good they were replaced last year due to leakage.

anyhow, im very frustrated with this because im not in a position to get a newer vehicle. Everything with my car is fabulous! Runs amazing! Still very stong motor. Im a very considerate braker, I dont crush my brake peddal to stop. Etc

im being told that my Booster might be at the end of its life and need replacment.
is this true? Or is there some other issue or part that would be causing this problem?
its getting costly on the pocket to pay for labour for replacing this.
desperate for a solution.

i used to apprentice so I havea very good idea about quality work and what to expect to happy from wear and tear. Or normal life span of parts. I have good faith in my mechanic, and im doubting its his hand at work thats the issue, as everytime ive dont some service, everything has been immaculate after, with the exception of this situation.
please help if anyone out there can!

Cheers!