Indicators of a malfunctioning brake pressure accumulator (brake bomb): the brake pedal exhibits rigidity during abrupt braking. Maintaining pressure on the brake pedal results in a gradual softening, enhancing the vehicle's braking efficacy; yet, the initial application of the brakes may feel ineffective. Additionally, you can conduct a test with the vehicle stationary and the engine off by applying the brake pedal forcefully around 20 times or more. The pedal is expected to feel somewhat soft, akin to regular driving, for the initial 15-20 applications, after which it should become firm. If it feels difficult initially or after only a few presses, the brake bomb is defective, indicating it cannot generate sufficient pressure.
Based on your posts, I am uncertain whether this is the accumulator/brake bomb.