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The operating pressures could and should be the same if the required steering box pressure is the same however the volume could be too low(not moving enough oil) for your original spec pump to do both steer and stop. It is in most cases with hydraulics when you add power users at the same pressure you will need more volume. I would imagine the Ford part number would remain the same if indeed there is no difference.
Just for an example, as you know if you put your thumb over the end of a garden hose you will increase the pressure by reducing the volume. Your issue may be caused because the pump can't move enough oil because with the brake booster (it increased flow but reduced restriction required for proper pressure) you do not maintain hydraulic (force) pressure required for both loads on the system(pump). It is just a thought, you may want to(if possible) restrict the return circuit (if used) from the booster and see if it helps. It may be possible the SD pump has a pressure regulator valve on the return side.
I am not sure about the actual exacution used by Ford for this system, just trying to help from what I know about working with various hydraulic systems for 30 years or so.
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