I picked up this cheap pancake air compressor at Sears 5 or 6 years ago. The little compressor finally took a dump so I turned the tank into a portable one and saved the motor. The motor still works, just doesn't compress air anymore.
I made this adapter from two quick disconnect fittings so I can fill the tank right from my air hose.
I just plug the fittings together and crack the ball valve on the hose until the little tank has about 90 PSI in it.
Turn off the valve and unplug the fitting from the tank. It fills pretty quick. It's only 3 gallons...