[Hydratech®]

Great! Now you can definitely rule out the MC as the culprit, and can continue moving your diagnostics further downstream in the brake system.

FYI - A bent caliper support bracket would be not readily visible to the untrained eye, and could cause the caliper to deflect crooked / cocked ever so slightly off plane in relation to the brake rotor. Then when you try to apply pressure to the caliper, you are "wasting" all of the fluid volume going to the caliper while the caliper straightens itself out by trying to clamp straight on the rotor. Take a close look at the calipers, pads, rotors to see if you spot anything ever so slightly "off" angle. Take a bright light and see if there is any air gap at say the front of the pads, yet no air gap at the rear of the pads (or on any other plane in relation to the rotor).

One glaring question - are the bleeder screws at the tops of the calipers? I just can't tell you how many times I have seen the calipers installed backwards (left caliper swapped with the right caliper, finding bleeder screws at the bottom instead of the top. You could run a tanker truck of fluid through the brake system and never get the air out if this is by chance the case.

Humor me - run up to the auto parts store and get some 3/16 brake line plugs in the brass section, then plug the rear axle flex hose where the steel brake lines go out to the calipers. Fill the MC up with fluid, leave the two ports open on the end of the brake flex hose, then wait for the fluid to start dripping steadily out of the ports on the rear axle brake flex hose (gravity bleeding discussed in our online article that I put a link up to in my previous post). Once the fluid is dripping steadily, then go ahead and pop the brake line port plugs into the flex hose and snug them up. If you then find a solid brake pedal, you have ruled out everything else in the vehicle's brake system except for the rear disc brakes (where this whole problem started). You could then connect one rear caliper at a time to the rear axle flex hose and bleed it out and see what happens. If everything goes bad again with say the driver's caliper hooked up (or vice a versa), then you can focus your attention to the specific area and then likely nail the problem.

Pneumatic testing: While potentially very messy and 'dangerous' to painted surfaces, you could also take a rubber tipped blow gun and actuate the brakes by hitting the brake lines (at the MC) with compressed air (use a large towel to catch any possible brake fluid). I know we are trying to rid the brake system of air, but push come to shove sometimes you do what you gotta do to figure out what in the heck is going on. With the compressed air, you can watch the brakes engage and release when you hit them with pressure, and might just spot something.