IIRC the spring is on the vacuum side of the diaphragm, Vacuum pulls the diaphragm against the spring pressure allowing fuel to flow. Once the engine is stopped the spring pushes the diaphragm to seal fuel flow. Remove the four screws and remove the cover. Place the spring on the other side of the diaphragm and replace the cover. Now the spring is pushing the diaphragm to the open position. Join the vacuum barb and the drain barb with a short length of appropriate tubing. Now you have to turn the petcock to the off position by hand.