All that has been posted in reply to your problem could be a help, but i had a small problem with something like that sorta symptoms. Turns out there was a foreign body that kept my brake boost vacuum one-way check valve from opening all the way. Air (vacuum) should only go one way. ALso, have you done the booster test? Engine off, pump out all residual vacuum in the booster so the pedal is up and hard, then with the pressure still on the brake pedal, start the car. If the pedal drops away, the booster is probably good. Hope this adds something.
Pat
2001 GV H25, auto, K&N air, BFG KO's (215/70/16). new KYB stock struts and shocks, Magnaflow secondary cat and muffler with 2.25 inch pipe