daxr

Very simple problem if you know how the master works. It has a return port for each of the two pistons in the master which is meant to be uncovered when the brakes aren't applied. That lets fluid return to the master from the calipers so they can release pressure and un-apply the bakes.

If the pushrod from the booster is too long, the return ports are never uncovered and fluid can't return to the master, and the brakes don't release. Getting the right booster/master combination is the right way to do it, but in a pinch it should be possible to cut a pushrod down to the correct length.