the vacuum gauge measures intake vacuum, not crankcase vacuum. I've stated this 100000 times on here, you HAVE to leave the pcv hose hooked up - this is where intake manifold vacuum is introduced to the crank case during idle/cruise.
there is a second hose from the intake (air cleaner box) to the breather also and this hose provides crankcase suction during WOT.
with a catch can, the only hose you change is the pvc valve hose. you do not connect both breathers with a t fitting. you simply install the catch can in between the breather and the pcv valve. the reason is you do not want boost pressure to get to your catch can.
the stock pcv valve is not designed for boost, and will leak like crazy if you run boost. this will pressureize the crankcase and blow out your dipstick and probably blow every oil seal in the motor.
The correct way to set it up is to use a more powerful one way checkvalve. I found that a standard brake booster hose works great, as it allows vacuum through but not boost.
if anyone would like a nice diagram, I have one that makes the installation of a catch can a snap.
hope this helps!
~Mike~