Brake Issue. Master Cylinder?

So, bit of back story. My 55 Wagon's brakes worked well. With the pedal about half way to the floor, I had full stopping power. One time, there was a pop/bang/noise and the pedal when to about 2 inches from the floor. The brakes still worked, and I drove it for a few weeks that way. I jacked up the truck and tested all 4 wheels and I was still able to spin the rear driver side tire by hand with the pedal to the floor. Once I disassembled that wheel, I found that they wheel cylinder wasn't hardly moving at all.
So, I replaced both rear wheel cylinders (and cleaned everything up) got everything back together and bled the lines with the vacuum bleeder...and.. was disappointed when the pedal still goes to the floor with zero resistance, and has basically no braking power, only enough to stop it from accidentally rolling out of the garage on its own.

Any thoughts on what my next move should be? I was thinking could be something inside the master cylinder. There are no leaks in the system as far as I can see, so I am not sure where else to go from here. Don't want to replace the master cylinder only to find out that it doesn't fix the issue either.

As an added bonus, I managed to snap one of the bleeder screws off the front wheel.
Ugh, guess I jinxed myself when I said I'd be driving it to work this week.

VTWilly

It was the master cylinder that had failed when mine popped and went to the floor.

If you got a good bleed on all 4 wheel cylinders, I think Jeff is on to it. The master cylinder is highly suspect.

Good news, after replacing the Master cylinder and getting a good bleed on the brakes, I am now back on the road!!! Got full braking power at about half pedal so life is good. Added bonus (I guess), I completely took apart all four wheels and cleaned everything and replaced all the wheel cylinders. This was mostly due to misdiagnoses on my part, and then breaking a bleeder screw, but still.
In addition, after some justification, I bought my new favorite tool for working on/restoring the wagon. I got a full size stand up blast box. I use a smaller bead blast media since my other hobby is restoring old lanterns and camp stoves (Coleman and other types) and I don't want an aggressive media for the lanterns.
It works awesome! I have pretty much been blasting and repainting any parts that come off or that I can get off. Brake backing plate, all the parking brake hardware, battery holder, brake pedal/arm, master cylinder bracket. It has been fun and also I like to know that things are getting restored.
This is so much cooler than just using the wire wheel, pictures below for the fun of it.

VTWilly

Great job VTWILLY. A sand blasting cab sure comes in handy around the shop. I believe we all have stories centered around the smell of something cooking over a Coleman camp stove. Great to know that there is someone out there bringing them back to life.

I have two of those Colman Camp stoves! What is it about cooking over white gas that makes food smell so good? You will use the heck out of the blasting cabinet. I have a similar one and an outdoor pressurize system. Nothing beats it when the rust, paint and corrosion is heavy.

And the sand blasting cabinet keeps all of the mess inside and keeps the shop cleaner; a selling point for the bride!.