You have a very bad and risky situation.
Either your new master cylinder is defective,
or you have a leak somewhere.
If you have a leak, there will be a puddle of
brake fluid under the leak.
New parts, even OEM are sometimes dead
out of the box.
When you push hard, the little bushing in the
peddle flares out and doesn't let fluid sneak
past. But a slight steady preasure will show if that
cup shaped bushing is bad.
I had a 1968 VW bug and I rebuilt a master cylinder.
I was on a budget, and was able to get the car cheap
because of the defective brakes. My dad told me it would
be simple to fix, but insisted that teh car be towed and
not driven to our home.
The old bushing was only about half the length of the
new one, and was worn smaller then the tube diameter.
Terry