I can use a bicycle pump or similar to evacuate my holding tank
offshore. I close both air vents, open the sea valve, and pump up the
holding tank with air. I watch the joker valve for back leakage / wear
indications, and now, use a 12 volt tire pump. When the pressure
reaches about a pound, out goes the poo.

If I did not open the sea cock, I could blow the contents out the deck
pumpout fitting...

This makes my list of the top 10 worst ideas I've ever heard!
Pressurizing a tank can result in a cracked tank, even a metal tank (I
know of at least one case in which the owner of a new stainless tank
didn't have enough sense to stop pumping the toilet against what he even
admitted was a LOT of backpressure till a seam exploded), blown out
fittings, or an eruption through the toilet...I've seen it turn even
brand new joker valves inside out. The pressure reading on your pump may
only indicate about 1 lb, but it takes a lot more than that inside the
tank to move anything. Sending the waste overboard is risky
enough..Don't even THINK of trying to send it out the deck fitting!

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/html/get_ri...oat_odors.html