The brake pedal sinks to the floor and have to push very hard for the car to stop. I can hear whooshing sound from the brake pedal area. No codes thrown but no communication with DSC module. Brake fluid is filled to max. All the rotors and pads are within spec. This started happening all of a sudden but been noticing the brake pedal slowly sinking to the floor when the car is started. Where do I start?

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'05 E53 X5 4.4i, '97 E39 528, '07 E92 335i, '16 F86 X6M.