Buzzing Brake Pedal various xantia's

[citroen al]

Hi guys
Over the years i have been lucky to drive hundreds of Xantias noticing that the brake pedal somtimes emits a buzzing sensation(not abs)..... obviously the brake doseur valve is causing the symptom however changing the valve dosnt always cure the problem but hydraflush/doseur valve does however on some occasions just replacing the LHM with Genuine Total stuff or indeed Castrol LHM also cures the symptoms...
Any thoughts !!!