Strip the master, you will find one of the jangle washers has dug into the cylinder wall and scored it. The piston then gets jammed at the bottom and won't return.
Strip it down clean it up, hone the score mark out of the cylinder and rebuild leaving the washer out and it will be fine.
Seems quite common, once you know you can take the washer out before you fit it.
It could be a minor alignment issue, but I bench tested a few and found the same problem