Just picked up a hasselblad 500c cheap but I'm having an issue of course... I figured out how to "unjam" everything but now, when I fire the shutter, the mirror and the rear curtain go up as they should but the shutter won't fire unless I push lightly on the upper rear curtain. Then the shutter fires and I can advance everything as it should. Is there something I can lube or adjust to fix this? Any advice would be appreciated!