we are basing our assumptions on the later abs setups, which have a separate master cylinder setup and a separate abs valve block

in which the master has one feed to the block for the rear line, one feed to the block for the front line and another return back from the block

the block then has 2 inputs and 4 outputs, one return to master, one line to the rear and one for each front

one the early versions the block was integrated into the master assembly

if you have the later one then you should be seeing pressure from the master to the block and it will be the output that is the problem, sometimes these have been caused by something as simple as an air lock