I just picked up a '90 Civic Si, and it has a DA rear disc conversion, and still has the stock Si 13/16" master cylinder/booster.

No matter what we've done, bleeding the brakes, bench bleeding the master cylinder, and checking literally ALL of the lines for flow, the front passenger and rear driver side calipers aren't getting any pressure. We've air tested the lines to each, and the line from the master cylinder to the proportioning valve, and they all checked out fine.

Since the problem corners both run through the top of the proportioning valve, I'm thinking either something may be blocking it, or maybe it's stuck. I've read that some proportioning valves, as a safety feature, close when a line is blown. Unfortunately I don't know the history of the car, so it's a possibility, and I don't know how the swap was done. Has anyone had any experience with this issue, or know of a way to fix it? I'm thinking I may have to replace the valve, but I'm trying to be sure this is the issue.

We've spent all weekend just trying to figure out what the issue is, and since we've gone through the MC and lines, and still have only two functioning calipers, it's frustrating. Any insight is a huge help. Thanks!
Adam