Quote from Frank McElroy
"Yes, you have a leaking valve. Likely a small amount of debris is keeping it from fully seating. When bringing in the slide hold the slide in button for a few seconds until you hear a pitch tone change from the pump. This might clear the debris from the valve seat. "
I used this method when closing the slides. Woohoo! It appears to have worked. No problem with slide extending during trip North from Florida.
Thanks Frank and others for this TIP