Joe- checked out booster- looks like it's a MICO unit- google their website & see if the booster there looks familiar.

Their boosters are all rated at between 0-2000 psi & between 3-10 GPM. Perhaps your challenge is flow- not pressure.

If your raise the engine rpm- does that change the symptoms?
If you hold the brake pedal to the floor (without pumping) will the pedal begin to rise on it's own at some point?

Hope this helps