This steel balloon is a high pressure nitrogen charge held by a diaphragm. The booster motor charges the system, and the accumulator holds the high pressure charge, until the brake pedal is used.
How do I know it's time to replace it?
You should check (replace), the accumulator if after you start the car, you ear the ABS booster for too long, say, 4 - 8 seconds. You can have air in the system leading to the same symptom, so bleed the brakes before buying a new accumulator. In my case, apart for the long time of the brake booster, every time i hit the brakes, ABS and brake warning lights lid and go out right after. Also braking was very delayed. Every time I hit the pedal, it was too soft, the pump kick in and after a short pump run, I had brakes again.
