Prefix the "val:" in front of the key name when removing a value from the session. Also, Clear out all values from local storage when doing a Session.Clear().

Without this fix no value was actually being removed from the session cache. This came to light when we turned on "SessionPageStatePersister" and noticed that even though we had the history set to 9, not a single viewstate was being removed. Works great now. 

Also, when doing a Session.Clear() the values were being removed from Redis but not the local storage, so any code that accessed the values further would still see old values. This removes them from both local and redis.