Kyle, long time since back at the BB. This is a good post and one which may point out some common issues with an aging braking system.
I had this same problem on my '68 144. The right rear caliper was clamping onto the disc without releasing. The issue was a clogged rear steel brabe line between the outer rubber hose and thbrake distribution valve. Years of crud built up. When I tried to bleed the line a tiny drip came out, removing the line from the caliper had the same result. When I removed the line at the distribution valve, I had free flow of fluid. After removing the valves, cleaning and replacing the steel lines everything was great.
Just take the time to work your way from the caliper back to find where the blockage is. It is most common to conclude that the valve is bad, but since much less fluid travels across the valve then the front braking system, it is more prone to having stagnant fluid and water accumulate, which cause the deposits.
Personally I wouldn't remove the valve, but replace it if necessary to maintain the correct braking balance.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon