i have a 1971 series 3 diesel swb with single line servo assisted brake system-the brakes are useless-when i baught the vehicle a year ago with a new mot they were bad then-since then i have replaced the whole system ie. all new-drums,wheel cylinders.adjusters,shoes,copper brake pipes,master cylinder and servo unit-everything that has been replaced is correct for the lwb system that was already fitted.
i have blead the system with an automatic bleeder from the furthest away to the nearest (osf) it makes no difference at all either doing it with the brakes fully adjusted or let right off-at any time press the pedal once will take the pedal almost to the floor will stop and does not pull to any side-pump the pedal once and the pedal is right up but takes a great deal of foot pressure to stop(my right leg is like popeyes) let the pedal go and its back down to the floor-there are no leaks and they are fully adjusted-the servo pump is driven off the fan belt and is fitted with a one way valve-please help as the mot is due soon

Do you have the right master cylinder fitted?

had something similar myself with the pull off springs incorrectly fitted.
also ajust each wheel one at a time.

to isolate a spongy pedal, the workshop manual reccommends clamping all 3 flexihoses- this should give a pefect pedal, if not then look at master cylinder. if ok remove rear clamp, if now spongy air in rear of system, if pedal still good remove one of front two clamps to see where problem is.
your servo system sounds like a later mod, so youre not using the inlet manifold or reservoir but a pump- is it giving enough suck?, can you feel the pedal go down when you start her up, there was a change in master cylinder size post 1980 , if you have the larger wheel cylinders then as ian said check you have the big MC