Audi A4 booster pressure not working (vacuum on manifold not working).

nes by their very design don not produce vacuum at the manifold like petrol cars do.
Mostly diesel engines use the vacuum pump that serves the brake booster servo,some of the vacuum is directed to the turbo boost circuit,then to servo pressure switches.
Start by making sure you actually have brake vacuum from the pump,
easy test is the booster test.
engine off,pump the brake pedal 4 or 5 times then hold it down as far a foot pressure can maintain,then while hoding pressure key on and start the engine,the pedal should dip lower under your foot,
no dip means no servo pressure.
Next test would be a vacuum gauge on the line from the vacuum pump when the engine is running,then it's a case of following the vacuum down to the pressure sensors/switch then the turbo dump valve.

hi I get audi a4 170 bhp some times the rpm goes to 1000rpm when my foot is not on the pedal any idea's what is wrong? thankyou

Have you had a look at this similar problem:

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