The MGB of the same vintage had the split master cylinder with the shuttle for the warning light which Graham is recalling. The master cylinder was integral with the servo.
The pedal on this arrangement was always on the soft side, but when pumped with the engine off, became hard very quickly. Start the engine and the pedal would sink and soften.
This isn't what you describe, but if that's the set up don't expect a hard pedal. My gut reaction is its the master cylinder seals. If they were time expired before the work, they will have travelled down the bore when bleeding the system, and may just have worn enough, or picked up some crud which is preventing a perfect seal.
It might be the servo - but just blank off the vac pipe and see if it makes a difference.
