Only enough for one good stop. 73 Duster

Well... it's showing 13 - 14" on the vacuum gauge. Not very encouraging.
Is this too weak to make the brake booster work properly?

EDIT: We put in the new wheel cylinders today. Old ones were, indeed, seized up.
Now getting some more braking power but this vacuum thing is really baffling.

The brakes work and feel great if you push down the pedal gradually. The car stops the way it should stop.

I cannot, on the other hand, slam my foot onto the brakes and instantaneously get the benefit of the booster.