The pump isn't "set" at a pressure, since it is an open center system. The pump provides constant flow, and will reach whatever pressure it can until a relief valve opens. The Aux valve (which the backhoe is connected to), was set by the factory at a relief pressure 3000 psi. The backhoe also has a main relief valve, which I adjusted just slightly to also be 3000 psi.
More importantly, bringing the relief pressure of an individual circuit higher will allow you to lock that function in place, and apply pressure using other functions. The increased pressure in that circuit is "disconnected" from the main hydraulic system until you open the control valve for that circuit.
The way I set the boom circuit to 4000 psi is by first backing off the boom relief valve, to decrease the pressure. I believe that I found that 1/2 turn equates to a change of 250 psi. From there, I brought it back to 3000 psi, then turned it in 2 more turns.
Edit: The reason I initially looked into this is because the backhoe functioned different than other machines that I have used. My neighbor has a 410D that I use frequently. When lifting with that, or digging roots or rocks, the boom is the least powerful function (as it is with the 75B). Once the boom is "stalled", you can lift or pry using the bucket or crowd, and the boom would stay where you left it. That would not happen with the 75B until I made this adjustment.
