I have a Mamiya M645 S1000 with an AE prism viewfinder. This viewfinder has a pin that should click into the aperture lever of the lens. Normally there is pressure on this that pushes the pin down and fixes it well in the handle. Unfortunately this does not work properly any more. Sometimes the pin is pushed down properly, but usually it stays pressed against the top of the viewfinder when a new lens is inserted and the aperture ring is turned to lock with the pin. It appears the spring is not working as it should anymore... Anyone have any idea how to open the viewfinder? And then further?