Audi A4 2002 1.9 tdi SE Brake Booster/Limp Mode issue

[mcgoual]

Audi A4 2002 1.9 tdi SE b6, 107,000 miles

Hi, i am wondering if anyone else has had a similar issue with an intermittent problem with the brake servo coinciding with generally poor performance, turbo not kicking in.
Its like the cars in limp mode and the brake pedal is really hard also.

The problem will go away after driving for an hour or so, or sometimes a restart will clear it, but after its parked up for a few hours or overnight the problems return.

There seems to be decent vacuum when i disconnect the hose at the booster and put my finger over the pipe, But i dont have another A4 to compare it with.

I have read many threads before posting this, seems to be falling in between threads, (thats why im posting) ie limp mode, and brake booster issues, but i think the 2 issues im having are related.

I did a scan with a hand held diagnostic tool and nothing is reported related to the engine.
There does not seem to be any leaks in the pipes, i did the vacuum leak test with soapy water etc.
The only thing i noticed is that the pipe going into the EGR valve was hanging out, because the retaining clip had broken off on one side which meant there was not a good seal.
Would this affect performance?
Its the fairly big pipe at a 90degree angle just above the pipes for the brake booster. Anyway i just replaced that pipe, seal and clip, issue still persists.

I read about check valves for the brake servo but cannot seem to find any part numbers, where its located....

there was also talk about some N75 sensor or something.

Does anyone think its the actual brake booster? Do Brake boosters fail outright as in catastrophically or can they fail intermittently?

any help really appreciated..

[zollaf]

you either have a vacuum leak or a defective vacuum pump.

[Ryan22english]

I have this exact problem but permanently I've got a hard brake pedal and no power also i have done a scan with a obd2 and it's coming up p0299 and p0401 I'm pulling my hair out with it as it got this bad over this month I've checked all vac pipes they seem ok I've cleaned egr which was in a bad way I've got an Audi a4 1.9tdi 53 plate

[Crasher]

Check harder, you have a vacuum leak. Take the 3mm vacuum hose off of the vacuum pump servo feed hose and suck on it, if you can pull air through you have a leak. A common failure is the N75 valve leaking.