Walter The flow stops because there is a 12-volt clutch on the engine. When you push the stop button on the gun, the clutch disengages. In turn stopping the pump. But allowing the engine to run on. With the pump stopped there is no draw from the pump. So there is no place for it to go. But stay in your tank until you push the on button on the gun. So the pump will start pumping and you will have flow and draw again. In our system the acid flows upstream when you push the #1 button, which controls a Solenoid valve allowing the acid to flow into the system. For the second step you push the #2 button, which opens another Solenoid valve allowing the soap into the system. Then push the #3 button and you can wax or etc.Ho it also opens a Solenoid valve letting it flow into the system. If you turn off buttons #1, #2 or #3 it cleans the system out allowing you to rinse off the soap or etc. The buttons only work one at a time except the on/off button, which has to be on to run the pump so it will draw from the injector/injectors .If you would like you can put the injector on the down stream side of the pump or even put one on the down stream side for acids and one on the up stream side for the second step and wax. Hopefully this is telling you something.
Terry