Some of the "B" vans had brackets that offset and even turned the master 90 degrees to the booster. All of those had some sort of a pivot assembly for the activation rod. Over time, I have seen those pivots gum up and restrict the free movement of the brake activation rod. Going down, you foot has enough power to overcome any resistance, but coming up the resistance can overcome the return spring. I have also seen where boosters were changed and the alignment between the master and the pedal bracket was off enough to cause a drag in the activation rod. You want ro be sure your problem isn't caused by either of these situations.

If the brake booster and its mounting are good, about the only thing that can pull a brake pedal down (or more accurately not allowing it to return to the top) is a faulty master cylinder.

The inside passages are partially plugged up and the fluid can not return to the reservoir. If you open the cap, is the fluid dark and dirty appearing? Flushing the master may work, but the odds are you need to replace the master cylinder. Gene