Steve, I’ll still stand by my doubt that the problem is with the brake booster, although it is likely that the booster could stand to be rebuilt to give you better braking power once you solve the problem.
The Booster is a Bendix model C hydrovac. I believe that ABS Power Brake in Orange, CA can rebuild yours for you or possibly sell you a rebuilt one. Their website is http://www.abspowerbrake.com/index.html
Moose