You just need check valves w/ low cracking pressure. These can be made as well. Define the pump as single acting and the problem becomes easier.Has anyone made a barrel pump from hardware store components? I have not been able to find plans or any information on this. It seams like it should be really easy and cheaper than the $200 commercial pumps.
The check valve that I used was a back flow prevention valve (for a garden hose) and it is in the base of the barrel so that air from the tube does not come out of the boat and into the pump. The back flow valve actually needed to have part of one side glued shut.Nicely done!
Where/what is the check valve you used?