1965 Other Pontiac Models brake pedal goes to floor after en

Brakes problem
1965 Other Pontiac Models V8 Automatic 180000 miles

pontiac catalina 65:

after engine start my brake pedal goes too far in before it starts to stop.
with engines turned off the brake pedal feels like normal.
i think my diaphragm check valve may be bad, cause I checked all the other possibilities like Low Brake Fluid Level or Air in the Brake Fluid.

i replaced some parts of the Master Cylinder too, only the check valve is an old one and I can ´t find a new.

do you have any other suggestions, where the problem can be?