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The PB line must be open and run to another open centre valve, to the sump or to the manifold aux system return port. The advantage of running it to the manifold block is that both the loader and 3ph will operate without changing the diverter valve. If connected through the manifold, oil returns to the sump through the 3ph, but you’re right, open centre systems always require an open path from the pump to the sump when no control valve is operated. It is possible to damage a valve assembly by reversing the PB and T lines. The T line handles low-pressure cylinder return oil, while line PB line pressures may go up to the system relief valve setting when a down-stream control valve closes the centre. If the PB and T lines were reversed, parts of the valve assembly that are built to withstand low-pressures may be subjected to high pressure if the loader and 3ph are operated simultaneously. The amount of pressure would depend the weight on the 3ph. I run my backhoe from a valve controlled pair on the rear of the tractor (the 3rd valve on my loader valve assembly). I make the connections and bungee-cord the valve forward when operating the hoe. Unlike ordinary double-acting cylinders, hoes have definite inputs and outputs. Connecting them backwards damages things. To make this type connection, I have to be certain which hose gets pressure when the control valve is forward. I coded my hoses with coloured plastic ties, but a pressure hose can be felt to stiffen when a control valve is pulled. Perhaps a better connection (since there are fewer valve parts in the line) is to run the PB line to the hoe input in which case the open centre flow runs through the hoe valve and out the hoe return line on route to the sump. A short hose can be made to connect the ends of the PB line together when the hoe is removed. If quick connectors are used for this hose, it must be well protected. Quick connectors pull apart easily, in which case there could be a lot of oil on the ground.
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