I replaced the seat post on my 84 Schwinn Voyageur SP recently, and now it frequently slides down while riding. I've made sure the bolt is tight, but it doesn't seem to ever hold completely. The seat post is 26.6, which should be correct for the year and model. I'm wondering if the bolt is a replacement, and isn't able to tighten properly due to being too long, or if the Voyageur's cable hanger could be the culprit. Has anyone else with a Voyageur (or similar cable hanger) had this problem before?

I've attached some cell phone shots of the seat post bolt — boosted the shadows a bit to try showing it more clearly.