What is this hose connected to my brake booster (pic included)?

[avanti284]

On my 02 GTI 1.8T (automatic) there is a brake vacuum leak someplace. A mechanic told me that it was the brake booster that was leaking but I replaced it and it didn't fix it (I replaced it myself...saved $650). Now I'm trying to find the actual leak source, and I notice that there is a mysterious hose connected to my brake booster that goes down and back behind the firewall in addition to vacuum hose coming from the intake manifold (see pic).

I cannot find any drawing or technical reference to the use of this tee fitting and mysterious bottom hose. When I suck on the mysterious hose, I only feel a little bit of resistance so that means that suction is being lost.

Can somebody please tell me what the hose going down is?

http://www.flickr.com/photos/7782568...in/photostream

[greyhare]

Not 100% positive but, I believe that goes to the vacuum pump.

[avanti284]

Not 100% positive but, I believe that goes to the vacuum pump.

I thought that the boost vacuum pump was inside the intake manifold, but I'm not positive because my repair books aren't very good. Anyways when I first bought my car, this bottom hose was completely disconnected. The brake boost was weak on startup and when I saw that hose disconnected i thought for sure that putting it back on would fix this but it didn't.

I still have that weak brake boost on startup (weak suction) problem now and it seems to be getting worse. A mechanic identified a vacuum leak but he diagnosed it incorrectly so I'm still trying to find it. As a side note, when the brake boost is weak on startup, it lasts for 45 secpnds or so, and if I try to back up from my driveway during this time I can hear a humming sound and the engine runs rougher as if loaded in addition to there being very little brake boost (very stiff pedal).

Does anybody have any thoughts on the cause of this?