Pentax 135/2.5 with malfunction?

[Henri Toivonen]

I got a good deal on a SMC 135/2.5, very cheap, but it doesn't work 
properly.
The aperture lever on the back isn't working. It moves but doesn't 
affect the aperture at all.
There isn't oil in there, it's somekind of mechanical thing.

Clues? Easy to fix? Worth the effort?
I would be getting it for some $20-25.
/Henri