Unless there are 2 or more bilge pumps using the same outlet, check valves in the pump outlets are not recommended. If one of the check valves fail, the other pump may well pump water back into the bilge through the failed check valve.
Most bilge pumps can only pump water not air. If the vanes in the pump are not completely submerged they will not pump water, they have no ability to lift water up and push it out. Diaphragm pumps can lift water and push it out.
The reason manufacturer's put check valves on their vane pumps is to stop water from draining back into the bilge. The vane pumps will push water out of the bilge and once the vanes are above the water level they will not push the water any further. The pump shuts off and water in the hose drains back into the bilge, only to repeat the cycle. Once the water level is low enough for the float switch to shut the pump off, water in the bilge will remain in the bilge until water rises enough to turn the float switch back on.
In a bucket with the pump fully submerged the pump will pump. If you let it pump all the water out and shut it off water in the hose will drain back in to the bucket.
As
@Don S/V ILLusion alludes to head, or how high the pump has to push the water is an issue. A long hose and a large head will allow lots of water in the hose to drain back into the bilge.
A deceptive marketing plan is to report the pump's capacity at zero head. The higher the head, the lower the pump's capacity. In real life the 750 gps pump will pump much less than 750 mph.
