On most model valves I run across what you've described is the diaphragm.
Thanks for your prompt help. The model I have are RainJet RJ711's.
If this is indeed the diaphragm, would it be easier and more cost effective to just replace the diaphragm, or should I just go ahead and replace the entire valve? I'd hate to replace the diaphragms only to find another problem and then end up replacing the whole valve anyways.