I had a similar problem with new calipers new pads new soft lines near rear shoes and new rear cylinders
my front calipers had to be hung on wire with the bleeder right at the top and bled with a block of wood in them. they were an absolute pain in the *** to get bubble free.
i also needed to reset my hub bearings as although there was little noticeable play there was enough to knock the pads back. reset to spec after new grease helped. just done as per the book.
and i had to bleed them with the rear brakes adjusted hard up against the drums, take them out of the equation to stop the brake safety/danger switch/idiot light triggering
once i had that sorted out there was still sponginess to the peddle I had to bleed the rears at the distribution block on the diff by snapping open each line a small amount while someone pressed the peddle and doing it up while fluid spurted out before they stopped pressing.
drove me mad for a week, mityvac pumping the fluid in from the bleeders with an adapter all kinds of mad stuff tried...
once done i had a hard peddle but my brakes didn't work very well
pulled the rears and found about 1 inch at each end of each shoe was doing the breaking
the rest of the effort was potentially just bending the shoe and material backing
the arc to the shoes was wrong..... and the drums were not worn out.
did not have a brake shoe cutting adapter.. (usually used on 1950s scooters and bikes)
so adjusted them up to rubbing the drum and went for a drive. pulled the parking brake one notch or so.... yes they got hot
after much messing about wearing away my new brake shoes, eventually got the full shoe to contact the drum....
finally had mediocre brakes....
standing on the peddle now caused the back to lock first and this is with 9 inch drums (early style ditch finder duo servo brakes)
the right hand side of the car would try to overtake the front in a skid in a really alarming way.
it was at this point i fitted my brake bias.... and what a revelation that was.
this meant for the first time in the life of the car i could put the brakes on hard in a manner that actually made the front calipers do their work effectively. i.e they got the pressure they had needed all along to work as the manufacturer intended... without the rears locking up...
i now have pretty good brakes
my problem was a combination of issues all conspiring to cause alarm at the same time,
and naturally you think its one issue and concentrate on one issue, until you get so annoyed that you start doing stuff that you didn't need to do.
the rear caused more problems than the front...
but it meant the front never got to operate at the appropriate line pressure to stop the car
Dave