The typical Audi brake problem

 

is a jamming parking brake lever in the rear brake caliper (Detail "B") caused by excessive rust. The lever does not slide any more back into its neutral position (Detail "A"). This results in the loss of the parking brake function. The driver usually notices this by a significant enlarged free travel of the handbrake lever. And basically such defective brake calipers will be noticed sooner or later at the next MOT.

 

Further potential sources of trouble are defective sleeves and stuck brake pistons in the calipers. The brake caliper support can be blocked by corrosion on its guide pins. In all these cases, the braking effect may decrease significantly. Due to a stuck brake, the brake pads and discs will wear out much earlier or even become damaged.

Front calipers

also show typical symptoms such as increased foot force necessary for braking due to stuck guides or pistons. Also knocking noises at the front axle can be caused by the brake calipers, particular if there is a lack of lubricant at their guides. 

Declining brake pressure and unmotivated self initiated braking maneuvers

are typical failures, showing a defective brake master cylinder. The brake master cylinder leaks  when the pedal pressure decreases during continuous brake actuation. It is also quite dangerous if the vehicle performs unmotivated by braking alone without operating the pedal. In this case there is probably a malfunction or significant contamination in the area of the central valve in the brake master cylinder. This valve inter alia ensures, that the heated brake fluid can return into the fluid container. If this is not provided, pressure may be built up in the brake circuit caused by heat expansion of brake fluid. This may engage the brakes with the result that the brake fluid is heated more and the process is further enhanced.

Sometimes in such a situation drivers may pour cold water onto the master cylinder, in order to eliminate this symptom. Anyhow, this does not solve the problem. Moreover water must never get in contact the brake fluid (reservoir), because brake fluid is hygroscopic.

it is not necessary to mention that any operation of a car with such error patterns is very dangerous.

The brake pressure accumulator (aka "bomb" or "brake– bomb")

is an extremely important part of the whole braking system. If it is losing its nitrogen gas pressure over time, the brake booster may fail suddenly. Then a much higher foot force is required to slow the car down efficiently. Since this effect can occur abruptly and unexpectedly, it is particularly dangerous.

Flashing hydraulic–pressure–warning–lamp after the engine starts indicates a loss of nitrogen in the accumulator sphere.