To recap - initial prob was spongy brakes (ATE Mk20 ABS system)
Remedy: fit new discs / pads - no bleed
Result: pedal goes to floor when running - hard when off
Remedy: 2 x new front callipers, vacuum bleed
Problem: rear w/cyl nipples rusted in
Remedy: replace cylinder/mechanism & shoes, vacuum bleed
Result: pedal goes to floor when running - hard when off
Remedy:manually bleed
Result: pedal goes to floor when running - hard when off
Remedy: fit replacement master cylinder, manually bleed
Result: pedal goes to floor when running - hard when off (I can see a pattern emerging....)
Remedy: fit replacement servo, fit replacement servo air pipes-engine, manually bleed
Result: pedal goes to floor when running - hard when off
Remedy: fit replacement ABS Modulator/main unit
Result: pedal goes to floor when running - hard when off

Note: when engine OFF and servo pressed - air is expelled/leaks fro somewhere (looking at front of car) on drivers side front of engine between extreme l/h side of block and starter motor somewhere up/under plastic moulding cover (not engine). This will happen with all 3 presses it takes to empty servo of air - but have no clue where air is coming from.
Took the car on road for 2 miles but had to return quickly as the pedal requires a monsterous amount of effort to slow (barely bring to a halt) when moving - once at a stop with foot on pedal it drops slowly straight to the floor - not halfway - straight down. At this point the brakes are applied and you can let clutch out and stall as it does not move - but let me be clear - the pedal is at the absolute bottom of its travel.

Conclusion - NONE whatsoever - totally baffled - anyone?

Sounds like you have replaced most of the system. Have you checked the flexy pipes for swelling under pressure. They should be changed every 3 years or so, most aren't.
Have you changed the ABS modulator? It could have a damaged valve.

Hi. As stated even the modulator has been changed. The flexis and hard pipes are all fine under pressure.

:d

So are you saying you have Brakes but lack servo assistance,in which case is the vacuum pump working properly.

Or are the brakes just not stopping the vehicle,as is even without servo assistance the vehicle should still stop but with more foot pressure.

I have a feeling youre brakes are ok but you are losing servo assistance.

Job done.....