You can purge the keg to get rid of air initially, but you have to leave the gas on the beer for several days before it reaches equilibrium. I will often charge the keg and take the gas off, then the next day there is no pressure in the keg because all the gas dissolved in the beer. Its takes several days of doing this befor ethe keg actually holds pressure. You can speed it a little by shaking/rolling the keg so more of the surface area of the beer is exposed to the gas.