1. MBM says you need 18" of vacuum, and you are less than that. 2. You're running a 9" booster, which is relatively small compared to the more typical 11" which provides more power boost. 3. Increasing bore size of master cylinder bore (your 1 1/8" vs 1") increases braking effort.
So my theory is that you have a trifecta of items, any one of which (and certainly all three together) could conspire to create a rock hard, unboosted pedal.
So I'm thinkin' you need to....get bigger diameter booster (and/or a canister), and/or add more vacuum with an electric pump, and/or or get smaller MC bore.
What does MCM tech support say about your prob? I admit they'd be a better resource than I am...and I also suspect they're gonna go with power brakes 101 as written right on their website....you need 18" of vacuum to run their booster...they 16" to 18" can be improved with a canister (again another clue that your 9" booster choice may be undersized for your amount of vacuum), and 16" or less will flat-out require an electric pump regardless of booster or canister size: http://www.mbmbrakeboosters.com/index.php?option=com_content&view=article&id=12:what-to-do
