Earl is a top end fabricator.... and his stuff is simply great. When I need another by-pass block ( the two I have are already spoken for on projects), it will definitely be an Earl creation. Get your valve checked or ensure with the manufacturer's data that your valve indeed does have a PRV. I initially thought that the Cessna Eaton valves I was using have an in-board PRV but I was very wrong on that count.
I flushed my system with Hy-tran believe it or not and then again with Perma-Tran. I didn't use a lot, just enough to make sure that it was clean. I think you could use just about any good petroleum solvent which is what the GSS-1024 calls out to clean parts etc., cause all you want to do is to clean out any trapped condensation from the system. Once the fluid is clear, it is flushed.
And now I understand what you are asking. Without looking at your tank all I can say is that there must be an out port ... possibly at the bottom/side of the beam. That would be where it the hose would lead back to the return port on the 140. This would also be where the filter would be as well... gotta have one of them. There must already be a hose from the out port on the valve to your existing tank which would be the return line for the valve. The pressure line would be from your existing splitter pump to the valve. Connect the pressure line from the 140 to this line/port and you should have your circuit. I hope that makes sense.
This one shows the frame tank, return line with the filter connected to the out port of the valve. The in port of the valve is connected to the pump/power pak. I am assuming yours is not a direct connect to the frame tank.
I found the thread where I did an upgrade on my auxiliary tank and it might be useful. Not Getting Much Seat Time, Lots of Time With Ellie Though