Ach, but I don't need to reverse the air flow. I install a splitter just before the balloon, and put on one end the pump, and on the other end a solonoid valve... fixed! The valve opens when water comes, the balloon under high pressure will deflate (will be a bit in the way of the water flowing, but not too much to hinder the flow I guess), water will be able to flow.
After 5 minutes I program the arduino to inflate again. Now I just need to learn to program arduino boards :p.