Kennyd is correct that a pressure gauge would have located your problem, but you would have to have made several measurements.
Here is why (I think).
According to my handbook, using ISO 32 fluid at 104 deg F, 8.5gpm through 30' of 1/4" ID hose with three right angle fittings has a pressure loss of 1215 psi. Add in a quick disconnect and a couple more fittings and you might get to 1250 psi. At 50 or 60 deg , or with heavier fluid than ISO 32, the pressure loss would be much more.
And that is for the one-way trip of 30'. The return hose from the other cylinder will have the same loss, for a total loss of 2500 psi if there were no resistance in the cylinders. So when you shifted the spool completely, so that 8.5gpm was directed to the 1/4" lines, the pressure in the relief valve rose quickly to 1500psi and the relief valve opened to limit the pressure. Therefore, the flow through the pressure line, cylinders, and return line was limited to a rate that produced a total pressure loss of 1500 psi.
The pressure loss was great because the fluid was moving so fast through the lines that its flow became turbulent rather than laminar, and the losses increase greatly when the flow changes from laminar to turbulent.
But a guage at the valve would have shown 1500psi since it would be measuring before the fluid entered the hoses. If you moved the guage to the cylinder end of the hose and deadheaded the cylinder supply hose into the gauge, you would also measure 1500psi because with the hose deadheaded into the gauge, no fluid would be flowing and there would be no pressure loss between the relief valve and the gauge. I think you would have to tee the gauge into the cylinder port so that the fluid could flow. Then the gauge would read much less than 1500psi because fluid would be flowing through the hose and suffering the pressure losses.
Glad it turned out to be an easy problem to fix.