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?