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Couple things here.
First, make sure your nipples on the rear brake pads are in the grooves on the caliper piston.
Second, you have to work the parking brake to adjust the rear calipers once they have been screwed back in all the way.
Finally, since the pedal is low, you may be able to determine where the air is by line locking the rear caliper hoses.
HERE is a link to the bleeding procedure that may help.
Please let me know if there is anything else I can do to help.
Ok then you need to lock the rear lines going to the calipers and see if the pedal is then firm. If it is, then one of the calipers is causing the problem. If the pedal is firm, then remove the line locks one at a time.
I am not missing the point, but don't expect the caliper to react the same as the front since A the front is closer to the master but also it's a completely different set up then the rear.
You have replaced everything right? With no success, this will help determine why the pedal is low.
Ok, let me know.
Ok, so, that eliminates the rear being the problem, work to the front calipers and do the same test. If you line lock all 4 and don't have any pedal, then it's going to be air in the master cylinder most likely causing the issue. Not to eliminate the ABS module just yet, but if you have all the calipers locked, then we know its not there. Basically we are piecing this to try and locate the problem.
Ok, appreciate the additional info. I assume the master cylinder was bench bled?
this tells me lets go after the master cylinder to start. If you have help, try this procedure.
Have the helper push down on the pedal, as they do this, you loosen the first line on the master cylinder. Have them hold it, then tighten it back up. Do this like 4-5 times on the rear line. Then repeat the procedure on the front line. Basically you are going to bench bleed the master on the car. After you do this, see if the pedal feels any better. Air is probably why there is low pressure at the rears. Sorry, it's a process....
Ok, Also, did you lock all 4 wheels at the same time or just the front 2 and the rear 2?
I see nothing special required to bleed this system, so it seems to indicate you have a bad master even though you have replaced it. If you would like I can open this back up for the other experts, but from here it seems like the master would have to be the issue. You could try bleeding it with the engine running and see if that improves anything.
Ok, just wanted to be sure. There has to be an issue there. Ok, hopefully this will open up for the other experts now.