Nice. I have been working on this but you beat me to the punch. Go ahead and market it to landscapers (guys doing chemical applications). That was going to be my market. Good Luck.
I never had any intentions to patent or market mine, but it was fun getting it working.
http://www.ptstate.com/forums/showthread.php?5324-Flow-Control-Valves-For-Roof-Cleaning
Strangely, there did not seem to be a whole lot of interest in my contraption back then. I made my system work, for me, but it was "tweaky". The suction tubes length from the pump to the tanks was critical. I used clear soft tubing to go to the tanks, and used what is called a pinch valve
over the top of the tubing. Very simple and cheap to do, and no leaks
Because the valve are numbered, the chemical strength of my mixture could be varied with good accuracy. The "Trick" to this suction based system was to make sure
both of the hoses primed. It was found that both valves needed to be open fully to allow both hoses to prime. If one valve was partially closed, and one was fully open, the open valve/tubing would "Hog" all the suction, and it would prime, and the other one would not. However, once BOTH of the suction tubes were primed, then I was able to restrict the pinch valves pretty much as I saw fit. And, I could simply clamp the SH Valve shut, to rinse my stuff out. Or, I could restrict the water valve, with the SH valve wide open, for a strong mixture to spray walks and driveways with, or for tile roofs. But I always wondered what chemical strength I was at, because I never added a 3rd suction hose to a soap tank, like Tim has done. That was a great idea!
I also thought about ways to improve my system, with check valves, manifolds, and vacuum gages, but here is what happened. My Men are on a salary + commission, and basically told me to "quite fooling around" with their trucks. Apple Roof Cleaning is mainly a roof cleaning company only, so our trucks carry 2 - 300 gallon pre mixed tanks, one is tile strength, and one is shingle strength. We have a valve to switch between tanks. We pull up to a job, see if it is a tile or shingle roof, and switch tanks accordingly. My Men told me "Boss, your new invention will only needlessly complicate xxxx, and slow us down" So, development stopped on this type of system, as far as I was concerned.
But, you know me, I am always thinking
My latest idea for a variable chemical delivery system is to use dual pumps, and a dual side by side hose, kind of like the welding hose used for a torch. Here is a thread about it
http://roofcleaninginstitute.org/topic/1178-a-variable-chemical-strength-softwash-system/