2007 Holden VZ Berlina

Had two disturbing incidents recently regarding my brakes.
Under normal braking car stops just fine.
When I brake heavily in an emergency the pedal almost goes to the floor, there's a grinding noise from the brakes and the car only just pulls up. Scary!

I had a couple of thoughts. Maybe the fluid is bypassing in the Master cylinder or perhaps there's an issue with the ABS unit?

Is there any way of testing or diagnosing which is which?

Thanks.