Reinhard - Mercury outboard motor dealers sell a spray made by Mercury I believe is called Storage Spray. You spray it into the cylinders of the outboard and crank the engine over a couple of times and it coats the inside of the cylinders to prevent rust and corrosion during storage. It leaves a film on the metal parts to protect them. Might be a good idea to spray the pump with this stuff before storing in a bag. A call to a Mercury dealer should answer the question if this is is good idea. I know I have used it on parts under the hood to prevent them from getting surface rust again and it seems to be holding 2 years later. Just a thought
Morris