Any reason a Brake Proportinoing Valve is not available?

[6cylinderstepside]

I am working on my brakes after the 79 has been sitting for a a couple years and the master cylinder is a bit leaky and the left front caliper is dragging. I have manual brakes and they are factory disc in front. the rears are fine. I decided to just replace it all in the front so I swapped out the MC and both calipers and the brakes are still dragging on the drivers side but release when I open the bleeder. Now it seems the culprit is the proportioning valve. The hose doesn't make sense as being the problem as it gets pressure and then releases with opening the bleeder. I checked the button on the valve but it's not popped up and the brake light is not on. I just replaced the tranny and did alot of work to the truck getting it ready for prime time, even the interior and it is ready to roll and this last thing is the only thing stopping me!

So that is where I am... I am about to try the ole open the bleeder on the good side and pump the brakes to center the proportioning valve again (hopefully) to free up the caliper but in the meantime I was checking auto parts store for a replacement and the usual suspects don't even show a proportioning valve listed to order more less in stock. If I clear the truck, parts show up, but when I add a vehicle it act like the truck doesn't even come with one but I am sitting here looking at it. Anyone ever heard of that or have anything else to try?

What gives with that?