Hi Chris, i havent installed an upgrade kit yet, but i do have a 200gpd ro-man system with their pump box too. I've just gone out to look at it and i cant see a one-way elbow on my setup, or any one-way fittings at all!!
Also im using a ro-man pump box and have just added a float valve to the tank but does it matter weather the solenoid in the pump box is before the di stage? , if this makes any sense
According to what i remember of the instructions that came with my pump box, you take the output of your pre-filters into the "In" of the pump and then the "Out" of the pump goes to the in of your 1st membrane. Then take the output from the membranes back into the box into the other input, ( for the sensor for cutting out the pump when your tank's full ) and then take the output to your tank and the float valve.
Hope this is clear enough. I dont have a di vessel before my storage tank but i think once you start using di resin you need to keep it damp, but i dont think it would matter where in the pipeline after the membranes you put it, just put it wherever it's easiest to plumb
Hope this helps,
Tim