Just a guess:

The float tanks have a drain fitting on the bottom which is always open to
the water. To lift, you pump air in from the top to force the water out. To
lower, you vent the air pressure to allow water to fill the tank.

David
dh@. wrote in message ...
How do the boat lifts that use air work? If they have a check
valve to let water out the bottom when they force air in, how
do they let the water back in to sink it? I thought maybe a
solenoid valve, but don't see any wires running to the float
tanks...only what appears to be an air hose. Is HydroHoist
the only lift that uses air? I read that they have a patent,
but they can't have a patent on the concept of using air
to lift a boat can they?