Ok, I am new to the car wash scene but have been a mechanic for many years. We have a D&S R/O system, one in bay automatic, 2 self serve bays and one truck bay. Whenever anybody in any bay selects RO then we have a good stream out of every pressure wand. About as much as the weep system puts out. When the automatic is on the RO rinse then we get more pressure at the wands. When we select presoak at the self serve then someone else selects RO it thins the presoak down drastically. I know it is a check valve problem but where would it be? Wouldn't it have to be at the RO unit itself since the problem is everywhere including the automatic that is self contained within the pump room? Any suggestions would be greatly appreciated. Thanks