Try this before you pull the aperture apart in your lens. It may be a simpler fix in the mount. This is not a guarantee but a quick check. If you see oil on the blades then you will have to pull the aperture. However this might be a simple fix.
This video shows how one can disassemble, repair, and reassemble these Promaster MC and Marexar CX MC 28mm 2.8 lenses. The problem is not always the aperture but more likely just the mechanism in the read mount that gets sticky from old grease.