I have a 1977 f150 power brakes. New calipers, pads, rotors, drums, shoes, wheel cylinders, brake booster, and new soft lines to calipers. The brakes are super touchy, if you barely touch them the rear brakes will lock up. Had it up on a lift and the front tires had a lot of resistance turning freely, cracked the bleeder on the calipers and they freed up. Could this possibly be a proportioning valve? The brakes have been bled several times trying to fix this but it doesn’t change. The vacuum brake booster gets 17 inches of vacuum. The old one had a vacuum leak.

 

Perhaps the rod/pin between the MC and brake booster needs to be adjusted? There's a thread on here about it somewhere. I'll try to find it.
found the instructions


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The rear brake shoes might be too far adjusted outward. Have you had the rear axle safely lifted with both rear tires off the ground, front wheels chocked, and try spinning each rear tires?
I adjust my rear brake drums until there is no drag. If you've already done this procedure, ignore the rest what I'll type. If not, remove your rear brake drums, turn the star adjuster inward retracting the brake shoes and then re-install each drum, adjust the star wheel lengthening the adjustment rod (expanding the shoes) until you feel a slight drag while turning the drum, then back the star wheel off a little until there isn't any drag. Know you indicated you've got new hardware, but if your adjustment rod isn't new, it will help to clean the threads and use a very light application of wheel bearing grease on the threads. Not trying to insult your experience,

 

A dirty gummed up metering/proportioning valve can cause this as well.

 

Sounds like you have too much residual pressure front and back. Cracking the bleeder on front and wheel freeing up is an indicator as well as rear brakes locking instantly. As MrGGermany stated, its easy enough to confirm your pushrod is set correctly. An quick easy way to check is loosen bolts on master cylinder at least 1/4-3/8 inch, you should be able to push it up against the booster with NO resistance felt and it shouldn't spring back. If there is resistance and it pushes back when you let go, the push rod is out too far and compressing the master cylinder piston which is not good. Check that and report back.
It wasn't in your replacement list but did you change the master cylinder too?

 

Another one ....