Answer:
Go past Sully's to the Living Quarters area. There is a robot crab by the door. If you jump on top of him, he flips over, and you get him and the key to Sully's.
Use the steam battery (from the trash bin next to the Clockwork Cantina) and you can open the door to Sully's : move the connectors around to equal -5 on the dial -- this turns the battery's input pressure of 10 down to 5, and opens the door.
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How to Use the Steam Battery
The small connectors running between the left and right sides set the pressure. The input is 10 (psi or whatever) and you have to change that to fit the door you are trying to open. For Sully's door, the output has to be 5, for the bridge 16, and for the Living Quarters 7. That means that the "total change" for all of the connectors has to modify the input by that amount.
Sully's door -- total change on the bottom meter is -5
Bridge control -- total change on the bottom meter is +6
Living Quarters -- total change on the bottom meter is -3
You change the total by rotating the valve connectors to include or exclude connectors from the path. For example, you need more "negative" valves for Sully or the Living Quarters, and more "positive" values (most of the +2) to bring the pressure up for the Bridge. You can practice rotating the pipes, but try to work from left to right to avoid having a disconnected (open) path. Once you see how the rotation of one pipe changes the pressure, you can revert it, or continue to make other changes down the line, moving up or down from the central pathway. When you have the right total change (-5, +6, or -3), press the steam release at upper left, or close to start over.