What is this?

Mattax said
JDMopar provided the first correct answer. While its true the automobile manufactures sometimes bought the same products from the brake manufactures, so that valve may have GM/Ford/AMC/IH applications too. My point is the metering valves were used on the mid size Chrysler products.

Metering valves hold off the initial pressure to the front brakes to reduce front wheel lock up on ice, etc.
MTSC 70-9 Brake Hydraulics 1970 Chrysler Imperial Hydraulic Brake Service Guide from the Master Technicians Service Conference Session 274

1972 Brake System Hydraulic Controls fo B-bodies
View attachment 1716265912
snip from
MTSC 72-4 Disk Brake Service Roundup, page 13
[URL]https://www.web.imperialclub.info/Repair/Lit/Master/293/page13.htm[/URL]

Regardless, since the problem is with the rear brakes, the problem is not the metering valve but something blocking the passages to the rear. It could be in the block or anywhere between the master and the wheel cylinder.

Thanks for the info. I'll put my prop valve back together after cleaning everything and I'll do that test to measure psi between the master and prop valve and outlet of the prop valve. There's only one brake line going under the car and to the "T" which then splits the pressure to either side right? So could I just blow air into the line that goes under the car and make sure that the air is coming out of both bleeders? If it is then the issue has to be with the connections going to the master/prop valve right?