I did not read the front pressure because the fronts are solid and seem to be working. Since I had the front working, no leaks, etc I did not want to open that part of the system up. Not really sure why that would tell me more? 800-900PSI at the MC and 300-400PSI at the caliper. Huge loss somewhere in the rear system and the combination value seems likely. From what I have read, the PV2 value will cut the pressure by 1/2 within a certain range. I¹m thinking I got sold the PV2 instead of the PV4 when I replaced it.
It is understandable that you don't want to open up the front since it is working. But I have made the mistake of thinking the line farthest from the booster is the rear and the other is the front, which was backwards (or vice versa, I have to trace the lines to tell, since I can never remember).
That's why I asked if you are sure the line you checked at the master cylinder is actually the line to the rear brakes. I don't know what can happen to cause 800 psi at one end, and half that at the other end. Doesn't seem plausible if they were the same line.
If you have the OEM type combination valve, at light pedal pressure you can have less pressure to the FRONT than the rear. And in that case, you could have less pressure to one set than is coming out of the master cylinder.
To summarize the thought process here: Something LIKE what you are describing COULD occur with an OEM type combination valve, and the lines from that valve that are supposed to go to the front going to the rear, and vice versa. But not the numbers you are reporting
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