Unfortunately, that diagram does not reflect how a Kubota is plumbed.
On a Kubota, oil is sucked from the sump by the pump and sent to the loader valve. The loader valve has a high pressure Power Beyond port which sends the oil to the next valve in the series circuit and a seperate return port line which goes back to the sump.
The loader's high pressure PB line now goes to any tractor mounted valve, which like the loader valve, has a PB out port and its own return to sump port.
The tractor mounted valve's high pressure PB line now goes to the 3pt, which is the last valve in the series circuit. The 3pt has no PB line and its return to sump port is internally plumbed.
If you have no tractor mounted valve then the loader valve's PB port goes directly to the 3pt. The 3pt is not connected directly to the pump but is the last valve in the series circuit.
Since a backhoe typically has no power beyond, it must be the last working valve in the circuit. Some installations have the loader's PB line connected to the backhoe and the backhoe outlet connected back into the main circuit but the 3pt and any tractor mounted valve must be disabled when the backhoe is connected. These type of installations will usually require that the 2 backhoe QD's be looped together whenever the backhoe is not attached.
A proper backhoe installation whould have a QD on the loader's (or even the tractor mounted valve's) PB line. When using the backhoe the backhoe's inlet would be connected to the QD and the backhoe outlet would be connected directly to the sump. This correctly disables any valve after the QD which are usually the 3pt and any tractor mounted valves. When the backhoe is not on the tractor, the line to the 3pt and tractor mounted valves would be connected to the QD. No looping of the 2 backhoe hoses would be necessary.
If you have a Kubota WSM there are nice hydraulic diagrams of the circuit.