S Type brake booster

Brake booster problem on 2005 S/type 2.7 diesel auto
Initial braking ok, but holding down pedal lightly makes next braking needing more & more brake pressure to get any braking


Soft pedal = air in brake system, failing master cylinder, failing hydraulic control unit, leak, etc.

Rock hard pedal = faulty brake booster, vacuum problem

If you have the soft pedal, I suggest bleeding the brakes with a diaphragm brake bleeder and the appopriate adapter. Completely replace all of the fluid this way. Then retest. If it’s not better, you may have a failing component