Just got a new craftsman air compressor (33 gal). I have read that it is important to "break in" the unit by running it with the pressurized chamber open (so that there is no load on the motor). How long do I do leave it running in this state? Is there anything else you would recommend I do?
Not so sure about needing to break in the unit, but if it is an oiled compressor then make sure you change the oil after the first 10 hours (or about that). Also if you do go ahead and break in the unit, some compressors have a duty cycle that is less than 100% (running 100% of the time). The compressor I have (Sears Craftsman 3 gal., 1 hp, Horizontal Tank) has a 50% duty cycle so it should not run more than 30 minutes in any one hour.
A new compressor should be broken in before being used for work. The compressor should be run for 30 minutes with the unloader valve locked open to seat the rings. The unloader valve can then be reset for normal compressor operation. The oil should be changed after the first few hours of operation. Check to see if there is any specific requirements; if there are no special requirements, use a quality synthetic oil.
I know from experience that break-in is required and a good idea for oilless compressors. I was unaware of that instruction once with a new one and sure enough it failed to reach its rated maximum psi after a single first success. From then on, it consistently came 10 pounds short. The instructions said run for a number of minutes without allowing pressure to build aka, drain valve open (I forget how many minutes...). I simply failed to read the manual! It did make a difference.