When trying to discharge overboard way out at sea I have success on one occassion and then the next time it doesn't discharge, and I then get a surging of air in the 20 gallon holding tank - my sailboat is a C & C 34 with the holding tank under the "V" birth. I've checked all of the hoses for clogs, and there are none. The thru-hull is open and works. The valves are set correctly. The pump (Whale Gusher 10) operates and there are no air leaks. The vent is open and there is no clogging. But, I only get success in discharging overboard once in awhile. Like I reported, it works and it doesn't work - I'm confused and I really would appreciate some help...thanks!