Nikon FM rear shutter opens on film advance return

[sphyrna]

recently took my Nikon FM off the shelf to do so shooting and discovered very bad light leaks on the processed negs. Either the neg is completely fogged out if I shot in daytime, or thick bands of white on the print, if I shot in lower light or shot in daylight and kept the lenscap on or did not keep the shutter primed. The light seals are going, but that's not the problem and I have ordered seals to redo.
The main problem appears to be with the shutter.

The camera has not been used in at least 4 years. I've been using a D40 and then a D90 but got the 'film bug' - I've had the FM nearly 30 years and up til now the mechanics have worked as if it just came out of the box.

I've uploaded some shots of the FM:
www.flickr.com/photos/22009203@N06/

Please note - all the actions are very smooth, nothing sticks nor is there any hesitation with the advance lever, and the shutter planes appear to move very smooth (the shutters were not touched by the way, the dark marks appear to be where the shutter planes have rubbed against each other possibly.)

1 is the camera at rest - shutter is not primed, mirror is down

2 is at the point of full film advance. The film advance is all the way to the right, the front shutter planes have lowered and the rear shutter planes have come down.

3 the rear shutter planes raise up as the film advance lever moves from the far right position back to the 'ON' at rest position. Looking under the rear of the lowered mirror, you can see through the lens - light comes in here and fogs the negatives.

When the shutter is pressed, actions are normal, the mirror flips up, then down and the rear shutter plane raises up.
4 is after the shutter release is depressed, the camera is back at rest as in picture 1.

I am confused as what is going on, something appears to be up with the shutter. Can anyone give me a detailed description of the normal actions for a FM shutter.

Appreciate any info at all.
Peter

 

[BobD]

3 the rear shutter planes raise up as the film advance lever moves from the far right position back to the 'ON' at rest position. Looking under the rear of the lowered mirror, you can see through the lens - light comes in here and fogs the negatives.

The shutter shouldn't open until the shutter release is pressed. The shutter should stay closed as the film advance lever returns after film advance.

If your shutter is opening at this point your camera is broken.