Re-read post #4...
Basically there is pressure created on the OUT line when the downstream valve is actuated that "could" cause the valve to fail.
Seems like that is what I said about the pressure on the tank outlet, and you disagreed.
Are you contradicting yourself about the pressure on the tank outlet port on the valve, as in post # 4. I have called the engineers at Prince, and also talked to several hyd technicians, and they all say the same thing. A lot of the valves I have looked at, give the max presure on the outlet port of the valve, and it is usually around 300 to 700 psi.
Although I understand that one can hook up all kinds of things, and they can work, but is not the accepted way to use the outlet port for down stream work. I know you have done it and it works for you, and may last forever. The only reference that I know of, is that the outlet port/tank port should go to tank, with little to no back pressure.
http://www.hydraulic-supply.com/pdf/328.pdf
I am kind of slow with this topic, but slowly I get there. By using the calculator I notice that the 3" bore at 2500 PSI produces ~ 17k lbs of push force. Don't know if this trailer will ever dump more that 3,000 lbs. The 4" bore cylinder at 2500 PSI is significantly more.
The OUT port is connected no matter what, without the PB plug it feeds the next valve in series (the 3PH in your case). With the PB plug it just dump waste out from the cylinders to the tank, and the PB port now feeds the next valve in the series.
So, if using the PB port, wouldn't it feed the 3PH given the Deere PB does not go back to the tank? Since the next valve is the 3PH, isn't it what the PB port connects to? If not, what is the purpose of the OUT port if I cannot go directly to the tank?