Fixed. Before I tell you what it was, I'll tell ya what I did. Now that I got the manual, I can see a pressure line heading back to the dump valve. So, first, I put a gauge on that line, back by the valve. Pressured out on my gauge at the pump, but next to nothing on that gauge by the dump valve. The quick disconnects on the plow block are gendered in such a way you can remove the plow and keep running the machine. The line from the pump connects to the line going back to the dump valve(that 3rd hose I was talking about), and the return you just tie it up out of the way, and put that dust cap on it. Ran that way just fine. Dump works, no pressuring out. So then I normalled it back up, and backed off that main 2300 psi relief all the way. Still pressured out. WTH? I decided to take the QD itself out of the equation, and connect the line directly to the block (had to adapterize a bit). Eureka! Of course, nothing at all happened because I had the relief pulled all the way out, but it ran and no pressuring out. I dialed some pressure back in, and everything works like a charm. That !#$%& quick disconnect was somehow blocking flow and pressuring it out. What a cluster! Anyhow, thanks all for trying to help. I've seen those quick disconnects leak, and not connect at all, straight up break, but never seen one be able to connect fine and intermittently block flow. This was a new one on me. Peace out.