You can make a check valve if you've got a BB and the type of fuel filter that unscrews. It has a little, fine screen in it. You unscrew the filter, put the BB in one side, and then screw it together. The BB should be on the side of the screen closest to the muffler. Or....the sequence of parts: 1. muffler pressure line 2. filter half 3. BB 4. screen 5. filter half 6 muffler pressure line to tank.
Won't work/run with it backwards.