[Racer26]

As long as you were only using one otherwise legal compressor at a time, I don't really see that you'd be breaking the rules to disconnect the on-board one and use an off-board one instead -- though I'm not sure what you'd hope to gain except a bunch of extra work (and perhaps save a couple minutes of run time on the on-board one). We're allowed to replace a compressor with a different one (the one and only one compressor rule doesn't intend to forbid us from replacing a failed compressor, but merely to limit us to exactly one compressor pressurizing the system), so I don't see that this sequence really violates the intent of that rule:

1. Build ROBOT with on-board compressor
2. Disconnect on-board compressor (as though to remove it and replace with a spare)
3. Connect spare compressor
4. Power-on test, let charge tanks.
5. Disconnect spare compressor
6. Reconnect on-board compressor.

As I said though, I'm really not sure why you would want to do this. Seems like a whole lot of extra work for no benefit.