I didn't see what year, model or mileage on the car in question but with past cars I have hunted spongy brakes. I thought it was master cylinder problems.
I then realized the flexible rubber brake lines were shot.
I replaced all four, bled the system and the pedal was rock hard.
Have someone push on the brake pedal while holding the flexible rubber brake lines. You should not feel them expand noticeably.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)