My Ultegra rear brake caliper is not springing into the open position like it used to. I've replace the cable and housing and checked the routing. Nothing has helped.
then I swapped calipers with my other bike and the problem still exsisted. I'm sure it's not the routing or housing or cable. The spring is seated properly but it looks like the plastic slider/guide that attaches to one end of the spring is broken. It doesnt appear to be interfering with the operation. Could this be the problem or could the spring be sprung?
I don't think the "spring is sprung" but it could be you need the tension adjusted. Attached might help http://www.parktool.com/repair/readhowto.asp?id=20 but since is advises on more than one occasion to "use care not to damage the plastic spring guide", the fact that your's looks broken might be problematic.
The plastic guide doesn't make that huge of an impact on the return-action. Most likely the caliper-arms are binding from stiction. Best to take the calipers apart and grease the pivots and washers. Then reassemble.