Yes a bit strange ,do you Know if the two pipes from the abs unit are supplying fluid or is one a return , the possibility that they are crossed over is something I need to check .
I have read that the abs unit has three pressure modes full braking half and none ,it would be fair to assume it is stuck on half power .
With the engine running ,brake pedal pressed hard ,you can turn the back wheels by hand but it takes a lot of effort .The front brakes are perfect though .I think the dual circuit brake operates a front and rear brake per circuit which must mean something if it's both back brakes that are weak . Incidentally all the calipers ,pads and discs are new .
An interesting puzzle ,any helpful ideas are greatly appreciated .
Cheers