It's my experience that the piston in the distribution block can move far enough in the bore to block one of the circuits. The piston has a groove around the center that the switch detent rides in. If the piston is forced off center by a large differential in pressure between the cylinders, the switch is activated.

You could remove the switch from the block and inspect the piston through the hole. I had Vanco Power Brake in CA rebuild mine.

If you completely remove the block and then remove the switch, you can remove the piston through one of the end holes after removing a seat.

I was going to suggest earlier that you had plugged bleeders, but if you removed them and closely inspected them, then I would look to the distribution block.

If you guys weren't so far away, I'd be glad to help out. I have a pneumatic brake bleeder that has always given me the best bled brakes.