Boxerworks.com

It's fixed.

Calipers were fine. The fluid return hole in the master cylinder was blocked. I pulled it, used spray brake cleaner, bled the brakes and it's working great.

If the brake releases instantly when you open a caliper bleeder, the return hole in the master cylinder is blocked.

Replacing the seals in the master cylinder(s) and caliper(s) on any forty year old vehicle is a not a bad idea even if everything is working well. EME has all the parts you'll need.

Exclusive tip for bleeding brake systems from the 'Mechanic from Hell" : (remember, you heard it here first)
Remove the pads and jam the pistons all the way in (I use scrap cedar shingle wedges) and then perform the bleeding.
You'll see how rock hard the lever becomes (virtually no travel) in a well bled system when the caliper pistons' movement is removed from the equation. Lever travel will return to normal when the pads are reinstalled and the pistons are again moving in and out freely.

Additional pro tip for the Mechanic From Hell wedgie technique: Have the reservoir nearly empty before you push the pistons back as the act of pushing them back will move fluid from bottom to top.