The spring on the aperture lever pulls the blades closed or rather moves the lever to the position to Allow them to close, down to whatever position you have chosen with the aperture ring. So if you have your lens set to f22 (off the camera) and pull the lever to Open the blades, as soon as you release it, they should spring back without delay. I don't remember about Pentax M lenses but Some lenses have 2 springs. The one you see on the top and another located in the body near the aperture blades.
IF the lens does not work as described as above, one of 3 things has likely happened.
1. The spring has come loose.
2. The spring has become too weak to do the job.
3. There is oil or some other lubricant on the blades.
In the case of 3, you may or may not see it but it will always prevent the blades from Closing or cause them to drag in doing so, not from opening. That's why lenses with stuck apertures will Always tend to Over expose photos.
6500