I will fix an appointment this week. We will measure the pressure in the hydraulic circuit (80-100 bar!), specifically directly after starting the engine. We will losen the pressure switch on the brakeservo and connect a gauge. This switch detects low pressure, and thats why you get the warning. I do not understand the vibrations in the pedal, you and I experiencing 5 secs orso. But soon I will know...
Your brake pedal feels hard when the pressure in the system is low (no brake servo, keine Bremsverstaerkung). This is always the case after 3-6 hrs stand still. But check your bomb (accumulator) by running your engine a few minutes (or after a ride), switch it off and you must be able to brake 30x minimum before you lose pressure->hard pedal. Please check also your hydraulic fluid level.
It is the same as the expansion drum (expansions Trommel beim Heizung) for your central heating system: the water presses away the membrane in the red drum, which is under pressure by nitrogen. So it prevents your radiators at home from exploding when the water expands....Physics from school, remember still?
Porsche 911S 1972