[Walt Fricke]

When you let off the brake, the springs inside the MC push the tandem pistons back. As the pistons move back they uncover a hole (one per piston) in the top of the MC. This hole leads to the reservoir. So as a result the fluid not only can move back into the MC from the lines to the calipers, but can also move up, if need be due to expansion, into the reservoir, or its connecting tubes.

If the connecting rod is too long, the pistons can't retract far enough to uncover these holes.

Since completely removing the rods pretty much guarantees that these holes are uncovered, what might be next?

The traditional issue was the rubber brake lines. With age, some rubber inside can become detatched and form a flap, like a one way valve, hindering the ability of the fluid inside the calipers from moving back toward the MC. But it seems improbable that this would affect all four calipers.

How about you try this: go make it lock up, and then open the cap on the reservoir. If, somehow, the reservoir vent is plugged up, this could account for your problem. The fluid trying to back up into the reservoir due to heat expansion pressurizes the reservoir.

Maybe. But you will know instantly if this is the problem.