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Discussion Starter #1 (Edited)
I have a very soft brake pedal and after giving it all rotors & pads all around and a fluid flush (which the car needed anyway), the brakes are still soft. No fluid is being lost

There is a hissing sound coming from the master cylinder area which goes away once the pedal is depressed. I'm pretty the problem is not the brake booster, after looking up threads discussing the symptoms

After looking at the diagrams, I see that the master cylinder connects to the brake booster with an O-ring and this is what usually fails. Should I just replace this O-ring, should I replace the whole cylinder + a new O-ring, or can I take the cylinder out of my parts car and swap it in with a fresh O-ring?

Any tips on the job?
 

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I don't think that that seal being bad would cause your symptoms.

Soft brake pedal would be:
  1. If the pedal totally does not sink no matter how gently or hard you get on it, and keep on it--air in the system
  2. If the pedal slowly drops, especially with light pressure--bad seals inside the master cylinder
My guess is #1. My 9-5 has a very firm brake pedal with short travel, and the shop says that's typical of 9-5 cars (versus NG9-3, where there's a lot of pedal travel).
 

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Discussion Starter #3
It’s actually #2. And if I come to a stop, foot relatively light on the brake, the car will actually creep forward after a moment or two.

So there are internal seals on the master that go bad then? I will replace it with a good one in that case
 
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