I'm purging the fuel system from the back tank to the fuel filter, and will eventually do the same for the front tank. I replaced the rear tank, filled it about halfway with fresh diesel, took off the fuel filter and am looking for the fuel to run as clear as the fresh diesel. I've gotten about a half gallon out, but it's still pretty red.

I am buying some biocide to put in, but how much fuel should I have to purge to get to the fresh diesel? The lines probably don't hold more than 8 ounces, and the same with the selector valve housing. Thanks.

 

What do you mean by red? Like dyed fuel?
Even though the lines don't hold much, dye can dilute well and still show up, at least thats my understanding on it.
Are you sure you have the switch on the right tank, and that the selector valve actually works?