[Claymore]

So, I recently bought a 64 T Bird that had been sitting for 2 years.
2 days after I bought her, the brake pedal goes to the floor and I have a fun time trying to stop.
The master cylinder was almost empty? 2days ealier it was fine? No obvious leaks...topped it up and they came back like new, so fast forward a couple of weeks and the same thing happened again except the master cylinder was ok level wise. I topped it up and the brakes still didn't come back?!
I've bled them... It got better, but now I get brakes sometimes, the next red light they've gone again??
Sometimes there is a springy thump and they come back?
The pedal is firm when the motor is off, but as soon as I start her it goes soft again?
I am fairly good with cars (late model stuff) but drum brakes have me confused
I have had everyone offer different advice like "if the pedal stays on the floor, your master is fine!"
What im asking for is proper advice, any 64 brake line diagrams,any cables under the hood needing tighten etc! Anything you can think of?!!!
I'm on a shoestring budget so any advice before I replace the master cylinder etc would be so helpful
Also the rear passenger wheel has steel brake lines but there is a cut hose up there too?
Is there a tensioner on each drum? Where if so?
Please help a poor Scotsman