Sounds like the accumulator is going out. Agree with the other post, you don't need to bleed anything in the assist system, but flush your brake fluid once a year to help with the expensive calipers and other brake parts. The booster system self-bleeding for the most part. Even when the booster is changed, or the accumulator. Make sure however, that you pump the brake pedal 30 times with the engine off to relieve the pressure before working on the system, like changing the accumulator.