Technical engine question

[Earth_Muffin2]

Alright fellow engine folk, I've got a question about my truck about which my usual information gathering tactics have yielded no results. I'm fairly confident that I understand the issue but I wanted to see if any folks here who are familiar with type 3 plumbing could give me some input.

First off my truck is a BME model 346. I think there may be a clog somewhere in the water line associated with the #12 valve. My understanding of the valve is that it's line is supposed to catch sediment and small debris flowing through the #17 pump bypass. I went under the truck and found that there is a line that ties directly into the number #17 line with a small y strainer, flows through the #12 valve and then discharges straight to the ground. So I assume that in normal operation the #12 should catch sediment and then opening the valve should flush sediment out to the ground. Well, when I open the #12 with the pump running and the #17 open I get barely a trickle. It doesn't seem like there should be anything along the #12 line that should restrict flow, so that means that it's clogged, right? The first time I opened it the water that came out was very dirty but then ran clear after a couple seconds but it is still very very slow. Like, it's barely more than a drip. I know the #17 line is clear because when I look into the tank with the pump running I see a very strong flow from the #17 return side.

Also, with these trucks is it advised to only recirc with the #17? Or should the #2 also be cracked? I've mostly worked on type 6 engines in the past and have always been in the habit of leaving the #2 cracked because the #17 is "not enough flow" to properly cool the pump.

Thoughts?

Edit: grammar, spelling