Mine only has one seacock since it uses fresh water to flush. I'd think you only need the discharge sea cock open to discharge. If the discharge pump is running and not leaking through the bellows, it is most likely the duckbill valves in the discharge pump assembly. There are two. They can be sucked backwards by pump out stations or can clog if not used routinely. Not too hard to replace, but messy. You can buy them on line from several places, as I remember about $20. The only other possibilities are a clogged discharge hose or bad vent. I found that the pump out cap on the deck must be on and closed for the discharge pump to work properly. The bellows or diaphram is pretty simple and you should be able to inspect it for holes. If it leaks you should see a mess around the top of the pump.