I was thinking about Lego compressors, and just compressors in general, and I thought about how much of a waste the return stroke is. There is almost no resistance from the cylinder on the return stroke, which made me wonder if it could be done faster, leaving more time spent on the compression stroke. As these things go, slower usually means more powerful, so a slower compression stroke could be advantageous. I actually ran into this mechanism on the web without directly looking for it. Check out these nifty mechanism animations here. So I built a quick return mechanism out of Lego, but have yet to test it in a compressor.
Update... I finally tried to build a quick return compressor, and failed miserably. See my pneumatics page.
The way this mechanism works is by a slider rotating about a crank. When the slider is at the top of the crank, it is also furthest away from the pivot point of the bar, meaning it will move the bar slower. As the slider gets closer to the bottom of the crank, it is also closer to the pivot point of the bar, meaning the bar moves faster. To use this in a compressor, you would connect a pump to the axle that is moved back and forth by the slider.