Distibutor Vacuum Advance
 

While checking for vacuum leaks to try and solve a rough idle and stumble when accelerating I could not find any until I checked the distributor vacuum advance. It does advance but will not hold a vacuum. The last one I had go bad on a Jag was totally open. Just want to make sure this is the problem before spending $111.

Since I will need to pull the distributor is there any thing else I should look for while I got it out? The car is a 85 500SEL with 160000 miles.

Thanks in advance.