This is what a properly function mirror box should look like.

The mirror release latch, mirror return latch, and shutter release lever all move freely. The bias springs keep them in their ready positions.

The mirror is charged by pushing the large post on the bottom towards the front of the camera.

Push the mirror release latch up to start the cycle. The mirror should move quickly to the taking position.

Push the mirror return linkage and the mirror to should move to the viewing position.