The old style pump has a hatch on the side: open it and pry the fuel injection element loose.
Be careful not to damage it, but the element is supposed to return by spring pressure. They sometimes get stuck in upper position.
Jerking the governing rod whilst fully topped up with diesel, is the first thing to try.
I never actually pried on the injection elements itself, just loosened it by jerking the governing rod on a running engine, through the governor inspection hatch. So apply sensible force, i havent ever tried how much these elements can take. (guess its ok though)