replace the check valves on the vac-lines leading to the brake booster and vac-canister. They couldn't cost more than $20 I would imagine, who knows? I recently installed a second (so now I have two check valves) in the line running to the vacuum canister and my brake pedal feels much much better. I think there may be a tiny leak in my check valve or who knows, but I'm not complaining about the killer brakes now. Hope you find it, that sounds like a nightmare!
Best ET 12.66 at 113mph & 2.0 60' (on bald street tires) Best Dyno 339 hp & 384 tq