Brian C. Miller's second paragraph is correct in terms of operation.
At the moment, and having noted your post that you have the non-TTL meter version prism, I am thinking something may not be coupling correctly irrespective of the prism/lens mount-dismount procedure. I would take the prism off and scrutinise the coupling chain; if it is broken (and a broken chain is conspicuous by its floppy appearance), that's a proper repair job. If it is intact, attention needs to shift to the coupling level in the lens mount. After those checks and operational tests, only then should the camera go in for service; because of this camera's engineering quirkiness, problems can and do manifest in unique and different ways — frustrating/infuriating ways, if you like. Never had this sort of problem with my 67 but then I read the instructions before I started removing things (just this afternoon I removed the prism top remove a stray hair that was annoying the crap out of me on the focusing screen — reassembled, loaded with Vaudeville Velvia and ready to roll).