I was afraid to say this... but has the master cylinder been in the car for long?
Here's what I have had happen to me... several times as a matter of fact.
The master cylinder will work great until you bleed the brakes. The bore of the cylinder is nice and smooth where the piston cups rub, but outside that range, it's kind of fugly. There may be a build up of sludge complicated with a ridge worn in the bore. Then you get in there with your size 12's on the brake pedal that goes all the way to the floor. The piston cups catch the ridge and that's all she wrote.
So, it's very possible that you may have a bad master cylinder. You will be able to pump the pressure up, but the pedal will still sink to the floor.
If it's not that... let's go with my favorite method for bleeding brakes and try that.
Starting with the farthest one and working your way to the closest.
Put an assistant in the car. My first wife actually got good at this. My second wife... not so much, but she looks better sitting there.
Have them pump the brakes three times. On the third stroke down, they hold the pedal down and yell "down" or I guess you can use some other catchy phrase.
Important--- They hold the pedal down until you tell them to release.
Crack the bleeder. I suggest putting a small rubber hose into a bottle to keep the brake fluid spray to a minimum. A clear bottle with brake fluid in it works great because you can see if there are air bubbles in the fluid.
Tighten the bleeder and tell your assistant to release the pedal. Rinse and repeat for the other three wheels.
The alternative method if you are by yourself is to use a hand vacuum pump with a bottle in the line.
Again... farthest to closest...
Connect the pump and get a little vacuum started. Again, crack the bleeder and watch the stream.
Hope this helps...