Stuck automatic aperture lever on Zenit ET

Just picked up a Zenit ET which doesnt stop down the aperture when the shutter fires. If I remove the lens, I can see that the lever moves slightly but appears "stuck". If I pull it out with my finger, it gets unstuck, and stops down the aperture as intended, but only until the next time the shutter fires, then it becomes stuck again. Any ideas of how to fix this?

It's a very simple hinged mechanism. You'll need to remove the top cover and the levers controlling the stop down lever go from shutter button to the right of the prism - there is an adjustable "slope" cam thingy there. (your fault point is most likely there). The big lever is hinged at left and right sides of prism.