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

This is not normal.  You either have a leak or a defective master cylinder.
Where did you get the replacement?  There are many shoddy rebuilt parts on
the market.

John

you have a serious problem.  while it's possible for the new master
cylinder to be defective, it's very unusual.  much more likely, you have
air in the system, especially as you've had the problem both before &
after a change of master.

bleed the system properly.  using the correct wheel sequence, have an
assistant pump the pedal hard 3 times, then after the 3rd stroke, with
full pressure on the brake pedal, you open the bleed nipple and have
your assistant press the pedal all the way to the floor.  repeat until
all wheels bleed clear unfrothy fluid.  pay close attention to the
reservoir level to make sure it doesn't drop too far.  don't scrimp on
the brake fluid - make sure you use a decent quantity to ensure the
whole system is fully flushed.