I have a older style PM rear master cylinder that continues to lose pressure. I've checked the lines, all are tight. No leaks as far as I can tell, taped threads.

I bought a rebuild kit for the MC, even after installing it my brake pressure gradually gives out until the caliper barely grabs. The fluid also turns cloudy grey pretty quickly. Kinda feels like the fluid gets dirty and then clogs up the ports.
I'm using DOT 5. It doesn't seem to lose fluid

Trying to figure out if the lines are just corroded on the inside and it's contaminating the fluid, or if the MC itself is shredding up the seals, leading to contamination.

The rebuild kit also came with an additional seal in the last picture. There wasn't one on the piston prior; just the one toward the front. I tried putting it on just to see but it looks like it blocks one of the holes from the reservoir.

Does anyone have an old MC like this or a diagram of assembly so I can verify I have the correct parts? Thanks.

 

Evacuate the system of all old brake fluid, and flush it with brakleen.