I can find many hand pumps for pulling a vacuum in a plastic sack, vacuum seal bags or in glass jars, but nothing that looks practical for 5 gallon buckets. The larger ones look like they are going to get into the 3 digit dollar range, which is not good at this time. The buckets are full of things like grains, cloth items, light tools and such. What I want is enough vacuum to firmly seal the snap-on lid down on the gasket.
I don't have the valve figured out, yet, but I think a flap valve style thingy would be fabricated fairly easily. Or, perhaps a standard tire valve, since I don't need a full vacuum, just some reverse pressure.