First, note that I used the most powerful motor. It is the RC Race Buggy motor (http://www.bricklink.com/catalogItem.asp?P=5292). I like it because it has the old style 9v connections which I can easily hook up to a 9v train regulator (http://www.bricklink.com/catalogItem.asp?P=2868b). This is necessary because as in real life, air compressors suck power in huge amounts. This system will eat batteries like they were M&Ms. The XL PF motor works too, but again the battery issue.
Using the train regulator obviously can't be done on mobile robots, so if you decide to use this on a wheeled or tracked robot, I seriously recommend using a dedicated battery box and having lots of spare batteries or keep the rechargables handy.
The manual pumps (http://www.bricklink.com/catalogItem.asp?P=2797c03) have to be modified, and for me this took a pair of needle nose pliers to remove the spring. I ruined the spring trying to take it off, but with with care it might be possible to take it off without stretching it.
It is important when building this compressor that the pumps are geared so that the pumps are 180 degrees out of phase with each other. This way, only one pump is compressing air at a time. Otherwise, the motor would have to work hard for a moment and then would have no resistance for a moment.
It is necessary to do some gearing and I used an 8:24 ratio. This gives plenty of power and speed. I also found that using the motor shaft nearest the end of the motor works best too as it has the highest gear ratio/lowest speed.