Brakes locking up

I've run across this on two different cars both doing exactly the same thing, get this:

At low speed, just off idle, the brakes are locked up. As near as I can tell it is the rears. Putting throttle to it rocks the car but won't let go. Switch to reverse and car will roll. Switch back and you get about a foot or two and it locks up again. I'm pretty sure it's the rear as it won't power brake where normally it can easily.
Had the same thing happen with a Duster that had a disc brake conversion also. Both were treated to new wheel cylinders and multiple master cylinders (73 Duster disc/drum cast iron master cyl's).

Is it possible the distribution block that houses the warning light has failed? As in the shuttle valve that senses between front and rear pressure lines gets stuck favoring the front and blocking the rear? Is that even possible, because in both cars it does release but not before your frustrated. Having it happen while driving is a whole other experience at a few stop lights and folks behind you not knowing what you're dealing with only add to the fun.