3DE

From the Porsche Workshop Manual (Carrera 2 addendum), even with the use of a power (pressure) bleeder, the caution in paragraph 2 states that the brake pedal must still be actuated at least 4 times. This is true for both the brake master cylinder and the wheel calipers. My question: does anyone have any technical information as to WHY the brake pedal should still be actuated while pressure bleeding? The caution is pretty explicit otherwise.