I have a 1984 Lancer 30 MK V. Having trouble understanding the Thetford Toilet. I am unable to find out how to open a valve to evacuate the tank. I assume that the pump handle in the port lazarette is for evacuation, but it does not seem to have any action (pressure on the lever). The Thetford manual has not been helpful, nor have the customer service reps. Can someone tell me?Thanks,George Haynie