Leaks
1) Missing pin in the sensor on the master cylinder can cause leaks around the plastic bolt over the sensor.
2) Rusty broken line under the master cylinder can fool one into thinking the master cylinder is leaking.
No LEAKS
1) Brakes can go the the floor with air in the lines. Bleed them.
2) Leaky booster. Still got brakes and pressure, just more effort to stop.
If the brakes are bled properly and they go to the floor with no external leaks, then the master cylinder could be bad. If new, the rubber seals were not installed properly or wrong seals, or missing?