Stop a lot of what? The only way the line could lose prime due to a lack of a foot valve would be if the gun was hung way above the jug and the solenoid was stuck open, unless someone blocks the sprayer with it running. If that happens the pressure from the compressor (Easily over 100 PSI) would just blow the foot valves off. The reason they're on chemical pickup hoses like Hydrominders is to keep the line full otherwise gravity would let it drain back into the pail, and having it fill each time would affect the dilution. As long as there's no leak, there's no reason the fragrance line would lose prime.