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Gerald, You are thinking of an "open-center" system, with a FLOW DIVIDER that continuously bleeds of a flow of a few GPM's for the steering circuit.
As this is a closed-center system, there is NO flow to the steering and brakes, unless they are being used, so the "priority valve" is NOT diverting any oil while the flow test is being done. (ASSUMING the steering and brakes are not being used at that time.)
The priority valve only "steps in" at low engine speeds, or extremely high hydraulic demands WHILE the steering or brakes are BEING USED, ensuring their oil flow needs are met BEFORE allowing ANY flow the the less critical hydraulic functions. There is an adjustment screw on the SCV that is analogous to the "rabbit-turtle control" on the newer tractors that sets to flow to the couplers. Have you checked the setting on that??? Also, a "high pressure leak to sump" will leak off output flow from the main pump, causing lower-than-expected output, and excessive heating of the oil. This could be in the steering, brakes, SCV, or 3-point (if used) systems. Generally, the component in the high pressure system with the "leak to sump" will begin to heat up more quickly than components that are functioning properly.
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