brakes issue

Hello,

my 2001 beetle is showing some strange behavior with the brakes.

At the morning I start the car and everything seems fine, after I drive for a bit, the brakes start acting weird.

The brake pedal becomes very hard (almost lie hitting a stop) at 1/2 of the way and you have to push very hard to get the car to brake.
If we look at the brake pedal stroke during this situation, after the initial play, you can still get the zone of very light braking, but after that it looks like the brake pedal gets almost a hard stop and you have to push extremely hard (with very little additional stroke) in order to get some braking.

Any idea of what the root cause could be?

Thanks