The pistons displace the brake fluid contained there. The pressure thus generated in the brake system is transmitted through pipes and hoses to the brake cylinders.
In them, pistons press the brake pads against the brake discs.
The liquid pressure is transmitted by two independent pipeline systems (brake circuits), moreover, for one front wheel and the opposite rear wheel (diagonally split dual circuit braking system).
If one brake circuit fails, then, despite this, two wheels are still braked.
The handbrake acts on the rear wheels via cables and mechanically presses the linings against the brake discs.
All Audi A4 models have had anti-lock braking as standard since the start of the series. More details about ABS are provided in the next chapter.
Tip: If one brake circuit fails, you should press the brake pedal much harder (the free travel of the brake pedal also becomes longer) in order to achieve a braking effect. The braking distance is also lengthened.
(The article is a reprint of material from: «audimanual»)