If you check that it has been piped up correctly, correct it if it's not and then make sure that you bleed the rear calipers with the nipple pointing to the sky then you won't need the Mondeo servo and master cylinder.

If the pipes have been replaced then you may not have noticed any issues before hand if they were put back incorrectly. You're only filling small front calipers with fluid with the standard setup so wouldn't have noticed as much until you fitted much larger calipers. This may have emphasised the issue that wasn't as apparent, or wasn't an issue before hand.

Like I said, my car is a S1 Turbo with standard pipe work and a standard master cylinder with AP CP5200 4 pots on the front and 2wd Cossie rears and the pedal is bang on. The 2 drivers side pipes go to 2 load compensator valves on the drivers inner wing then from there to the rear. The other 2 pipes go directly to each front wheel. My issue which was the same symptoms as you're having was just a small amount of air trapped in the rear calipers which only came out when I swung the caliper to point the nipple upwards.