Brake pedal spongy, hissing when depressing pedal

[The Norwegian]

Hello all,

I've got a 2002 2.0 ALT that I quite like, but it has a lot of vague problems. I've owned it for about a year, and the brakes have always been quite bad, but recently they've become even worse.
The actual brake force when braking for your life is pretty acceptable and it passed MOT, but I feel like you really have to work for anything to happen. Especially when stopping downhill, I don't think it's working as intended.

The rotors and pads should be OK, passed the tests and I have inspected them. No remarks on fluid leaks, fluid was changed sometimes last year by a shop and has been every 2 years of the cars lifetime.
There is a "whoosh" sound when pushing in the brakes, definitely coming from the pedal area. Like I'm physically pumping air somewhere.
The brakes feel spongy and will go to the floor if held in during idle.
The fluid reservoir is full and the level seems constant, even though I thought it appeared slightly overfilled. Not sure if relevant.
My vacuum system has an error P1479, which has to do with vacuum assist motor. I have plugged the hole and disconnected it, seeing as these are only on auto tranny cars, it shouldn't affect this. Vacuum system has an idle absolute pressure of about 350 mbar. Not sure if that's good or bad.

Should my first line of action be to re-bleed the system? Can this be caused by the vacuum system?

Thank you for your time!

[Rob69]

Sounds to me like the servo is faulty, either leaking diaphragm or sticking valves, sinking pedal likely to be master cylinder seals leaking.

[Alan02]

A faulty Servo can also cause the pedal to sink when depressed and held. Do you get a continuous hiss from servo on pressing pedal, or just a short hiss ? If continuous, its defo faulty. Try holding pressure on pedal without running the engine, it shouldnt sink.

[The Norwegian]

Thank you for your replies. I do think the servo is starting to wear - but is it dangerous driving with it? Can sudden complete failure occur?
I have fixed some vacuum leaks, causing the brake pedal to have a significantly better feel and power, so I won't bottom out the pedal in any scenario other than actively provoking it. Therefore I think I'll leave it to be for now.

[Alan02]

I wonder if that is a coincidence, as if you havent done any repairs at the servo itself, i wonder how the pedal now feels firmer?