64 Brake issue

[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

[Frat_man-du]

I had a simialr issue on one of my '65s. Loosen the bolts on the Mast. Cyl and gently pull out a bit - see if any signs of fluid leaking into the break booster if so the Master needs to be redone, and maybe soon the booster, if fluid got in there, not too likely as there is a lip and seal around the push rod to prevent this. I let mine go too long and got fluid into the booster - almost 10 oz was quite a mess...

Yes there is a rubber flex line on top of the Axle slightly towards the pass side, if I recall right. Just follow steel line along the top of the axle and you will find it. Silly easy to change. Just be careful when being under any car.

[Claymore]

Thanks, ill check into that.i'll let you know either way.
The line under the car looks like a vacuum line and has been gathered up, folded back on itself a couple of times and zip tied and is obviosly cut. But it's zip tied to the frame right at the passenger rear wheel arch?!

This is the first 60's car I've had, I actually went from a nissan 350z that I modified and worked on to the T Bird. It's 8 years older than me, as I was born in 72!!
I'm so glad I have it though as I always loved American cars when I was in Scotland (moved here 9 years ago)
It's amazing the stares and head nods I get here in Newport Beach, normally by guys driving cars 10 times more expensive!!! LOL

[Frat_man-du]

Ah...I suspect what you have found is the rear axle vent hose. on the pass side of the axle there is a nipple sticking up on top - the hose goes here. This hose lets it vent and to keep dirt from getting in. Might not be a bad idea to change the fluid - I actually have this done for me - to friggin messy for me well worth the $60.

Birds always turn heads, you cant get this kind of style and class from any new car - period.