Hello everyone, thank you in advance for reading.
I'm currently working on a 2003 Range Rover Vogue with the 4.4 Petrol Engine (53 plate). The owner recently replaced the rear subframe with one from a 2002 Range Rover 3.0 M57 (52 plate) and used the brakes and some suspension components from it.
Since the swap, there has been a persisstent issues with the brake pedal consistently being spongy every time the vehicle is started, no matter how thoroughly we bleed the system using both manual pedal pump and a brake bleeder putting over 4 litres of brake fluid through it before we had a pedal at all.
Interestingly, if we clamp off the two rear flexis, the pedal returns to a firm and safe state, this leads me to believe the issue is isolated to the rear braking system. I've noticed the rear calipers are differing from the original fitment with the right rear part number currently being SOB500042 however, according to diagrams it should be SMC000200.
My question is could using the wrong caliper here cause this brake pedal issue ?
Thanks you.

came across this problem twice, not on a rover
they had put the calipers on the wrong side
left on the right side
right on the left side
this put the bleeder at the bottom of the caliper and the hose at the top.
it couldn't bleed the air
good luck RR 2005 4.4 petrol
Any spelling mistakes are the fault of spellchecker
Previous MG J2, CITROEN light 15 x 2 gone unfortunately
Present MERC 180e, RANGE ROVER L322, JAGUAR XJS, MERC 280SE, MG F, JAGUAR S-Type 6v 2003, jaguar s-type 8v 2004, Ford Cougar