I seen that you plan on converting your excavator to air. I am interested in seeing how the hobbyking cylinders you ordered perform. If i recall correctly from back in the days on rcmt and rccrawlers they tried those air cylinders in place of axle mounted servos to mimic real monster trucks with axle mounted hydraulic steering. Their performance did not compare to servos.
But i did find these today... 100psi capable, 4" stroke
http://www.amazonsupply.com/dp/B005J...sp_dp_g2c_asin
I think you were talking about a pump or compressor like this one from harbor freight but it seems like it would be too big and not powerful enough.
http://www.harborfreight.com/12-volt...ssor-4077.html