The reservoir is........... just a reservoir............ just like the big chamber on your current booster. Adding one only extends the number of stops you can make on a stalled engine. It does not increase your braking performance one little bit on an already properly working system.
Your biggest enemy is the combination of; an Ica around 72*, in a 9.0 engine,with log manifolds. I have no idea how you coaxed 14/15 inches of idle vacuum out of it, unless she's idling really high.. or has Rhoads lifters.
Ima thinking to get a second opinion as to that vacuum number.
As Blu already said, the 15/16 M/C is a nice size.
