BRS 5 stage RODI setup question

[Charley75]

So I bought this system used and had some setup questions. So I see what I think is the flush valve on top. This system has a second valve between the 2nd and 3rd membrane (in red). What is this?

 

[Charley75]

Also noticed pressure gauge not working. Is it suppose to always be active? So after flushing with the top valve for a while, I switched it over to produce clean water and got 12 tds before di and 1tds after di. Should I buy a complete membrane replacement kit or just replace the di?

 

[dadnjesse]

That's to drain the higher Tds water. where does the other end that you can't see go.

 

[Charley75]

I think it goes to the flush valve.

 

[Charley75]

Still not sure