HI,

Just looked at your post again, and see you have tried master cylinder,

do you still have the RS brake load valves at the rear end, if so when you bleed
the rear brakes have the weight of the car on the lower arms to open the valves.
but does sound like the master cylinder seal are faulty, servo just assists the brake, less foot pressure needed, turn the engine off then pump brake peddle
and keep your foot held on and start the car the brake peddal with your pressure
on still,should go to the floor then as you let the peddle back up it should be hard as you put your foot back on, if servo good?

DAVE R