Overboard Discharge Help for 2005 Chaparral Signature 290
 

I purchased a 2005 Chaparral 290 Signature early this year and I am looking for help on how to operate the overboard discharge.

The head has a Waste System Level Indicator but it only shows "Power". Is this normal?

I tried running the overboard discharge by opening the discharge valve and pressing the Vacuum Flush button in the head. I can hear the vacuum working but nothing is discharged. There is a slot for a key next to the control for opening and closing the valve but I don't have a key that fits that slot and will turn.
Any help on how to get the overboard discharge to work is greatly appreciated. Thanks in advance for your help.