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The bucket control has a 'regenerative' position. This applies supply pressure to both cylinder ports. You'd think this would stall movement but, since the two sides of the piston have different areas (due to the rod on one side), it doesn't. Instead, the cylinder extends faster than it would normally, but with less available force. This lets the bucket dump faster and the full force isn't needed to dump. That regeneration works with cylinders but not motors, they stall.
Here's where I'm not entirely clear. There seems to be another position that does have double-action. It doesn't have a detent like the float position on your boom, but by pushing the lever only part way (or fully over, here's where I'm not sure) you should have the flow condition that will allow your motor to reverse.
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