Started firing up the systems on the boat only to find the forward toilet would not build a vacuum. I push the flush pedal, the bowl opens, water flows but just builds up as there is no vacuum in the line. I hear the chug a chug sound so the unit is trying to build a vacuum but just goes on and on. I let it run like this for a good 15 minutes, shut her down and tried again for a good 15 minutes but no joy. Can someone point me in the right direction before I rip everything apart unnecessarily.

Additional info: Worked perfectly fine last fall before winter layup and never an issue. No lines were disconnected or valves closed for winter layup. My procedure is to make sure the holding tank is as empty as possible and since the toilet draws water from the potable water system I simply flush the toilets while connected up to the "pink stuff" until I get a good stream of pink. Flush once and repeat a second time.