I've bled many a brake in my lifetime, and I can't put my finger on this one. I'll try to describe as best I can and see if anyone else has solved this riddle before. This is for a new brake system on my 41 Ford Pickup

My Master Cylinder is located under the driver floorboard, mounted to the chassis. I have a 4 port MC, but only used one side and plugged the other. I did a bench bleed with a kit and bled until no more bubbles on the two open ports. I reconnected the lines (closed to the mount for the front, furthest from the mount for rear) and bled the rear brakes first. No issues and both sides of the rear bled until clear fluid came out with no bubbles.

Here's the problem. I get no air, fluid, anything when attempting to bleed the front. When I break the front caliper valves loose, there's no relief of pedal pressure. I disconnected the lines to see if there was an obstruction, all the way back to the primary connection at the MC. When I disconnect that one, there isn't even any fluid dripping out of the open port. So, I thought maybe the MC was bad, but I did a bench bleed again, and it is definitely blowing fluid through the rubber hose back to the reservoir. Reconnected, and same issue. I have a bleed bottle and rubber hose that I connected to the driver's caliper, and opened that valve (with fluid in the bottle). With the MC cap off, when I pump the brakes, it blows bubbles in the bottle, so no obstructions. But as soon as I put the MC cap back on, and try to bleed normally, nothing. The pedal stays firm, so there's no relief when opening the valves.

I'm open to any and all ideas. This one stumped me.