where to buy stock brake proportioning valve? Brake problems.

I have a 74 duster with a power brake setup. I seem to never be able to get my brakes to work as good as they should. Car sat about 10 years and the brake fluid somehow went dry. When I first started working on getting the brakes working again-something was froze. I started car up and stood on brake pedal-at that time was manual brakes but still disc/drum. Finally something let loose and brake fluid was able to be bled from front calipers. Since then I have replaced rear brake wheel cylinders and all springs etc-all in one kit. Shoes were fine yet. Up front I replaced calipers and brake hoses. Also put on a used power brake booster and a new master cyl to match. Since everything that touches fluid is new- I used dot 4 synthetic. Booster seems to hold vacuum.

My brake pedal doesn't come back up by itself. Car stops ok but pedal is soft at first. Have to use toes to bring pedal back up. I have bled the crap out of the brakes. Im thinking that all those years the system sat dry that maybe the proportioning valve/switch is what "let loose" to let me bleed brakes and maybe that's the cause of all my evil because maybe the valve inside isn't working or sealing properly and maybe not getting the fluid where it needs to go. Maybe the valve pitted inside and fluid isn't being proportioned right?

I have searched a lot through the archives here and will try a few things tomorrow- most of which I have already tried- but hey- why not test again?? I only found one archive on here that mentioned the proportioning valve and Im thinking that's my problem.

Any ideas?

Anyway- back to the original question- where can I find a new proportioning valve/switch at? Rock auto doesn't list them, advance auto doesn't either. Did a search online and not having much luck yet. I think the right part number I need is-3466793, atleast that's what I am taking from a few pics n the archives.
Sorry so long for a valve lol. cant remember what my vacuum was when I checked it last year- 14? 18? Anyway- its a 360 motor with about 120k miles and a new stock cam and lifters about 2000 miles ago. Had this problem before new cam install also- so no differences from old cam to new cam.