It's a power brake car (with enough vacuum to run the booster).

The more I've looked around I'm pretty convinced I'm going to recommend switching to a "new" style 2 bolt 1 1/32" bore MC. From Dr. Diff's site this is the one recommended for 4 wheel discs. Then add an adjustable prop valve. I can't for see that combo not working.

I looked up the caliper bore sizes and they are pretty big - I just don't think it's pushing enough fluid quickly enough; and odds are it's a smaller bore MC right now on there.