Another way to check the servo is to push in the diaphragm and put your finger against the dump servo. It should hold the diaphragm until you remove your finger.
At this point, Chris says you should have found the problem, and that hes never replaced the instrument cluster due to a cruise control problem. If you feel your cruise control may be the problem, Chris suggests you purchase a shop manual for your Corvette before progressing any further. We got our 85 shop manual through Mid America Designs. Youll be dealing with electrical systems directly, and the potential for doing damage increases greatly once you remove the instrument cluster. If you dont have a shop manual and arent comfortable working on sensitive electronics, seek out professional help.