Brake troubles

Just rev it up a bit and you'll have plenty of vacuum.
If it still acts the same, you probably need to adjust the pushrod a lil shorter, or maybe longer
I start with shorter cuz if you make it too long you could break the control-valve. For shorter, just loosemn the nuts a few turns and slip some shims in between the booster and the m/c. If no change or worse then remove the shims,lengthen the pushrod one or two turns, and try again.

FWI: on a factory set-up, the frontmost reservoir is plumbed to the REAR brakes, through the Combination Valve; which has a Proportioning valve in it, and a safety switch which gets wired to the (!) lite or the (brake) lite to indicate a brake failure. If you have excessive pedal travel, like almost to the floor, this indicates a failure in one end, or the other; you have to figure out which it is.