Tape up the tube to start with, it wont last long but its a good test. If you have an aftermarket BOV put the stock one back on and take it back to service.
Im pretty sure that line should be under vac.
*Edit* the big tube or the little tube?
I dont think this is the source of the hissing...
with the TMIC clamps and hoses, did you just check if they were tight, or if the hose was on properly? - my clamp was on tight but the hose had slipped off a little on one side and it was not initially visible.
MY16 WRX STi Crystal White Pearl
-= Rally Armour Flaps - 2XS Muffler Delete =-
FG FPV F6 MKII Vixen - SOLD
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CX9 Grand Touring Stormy Blue
-= The rolling armchair =-
6 MPS Velocity Red - SOLD
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3 MPS Aurora Blue - SOLD
-=PG SRI - PG Inlet - ETS TMIC - GFB Hybrid BOV - Best Mufflers DP - Eibach Pro - AccessPort STG2 - Autotech HPFP - ITV22 - Michelin PSS=-