My first guess would be something in the power brake booster. Maybe disconnect the vacuum line to the booster and see if that makes any difference. If the MC is new and all 4 are locking up there isn't a lot left in the system. Is there a proportioning valve in that system that splits the fluid when it goes from the single line out of the MC? Is the MC brand new or rebuilt?

I used to own a 63 Bonneville, but it has been so long I forget what the system looks like. The more I think about it I'm betting bad booster.

Don