Release valve does not slide easily

We have the usual Thetford recirculating toilet in our 2006 2619. Over the past year the release valve has become very difficult to slide. We have tried WD40 but it does not seem to make a difference. Is taking the toilet apart and replacing the valve our only option? Because I think the valve is stuck in the open position, I'm almost sure the waste is going directly to the black pipe. There is only liquid waste which I suppose is a plus.