freon! add or not to add

wanting to add/check freon in home ac unit. hook manifold gage blue hose to low side port and yellow hose to r22 bottle. There is a valve with brass cap on it with a hexhead screw in it that adjusts with an allen wrench. do i have to adjust or screw this hexhead in or out to add freon? when i screw in the hexhead on the larger line the unit begins to sound different. i guess it is cutting down on the suction, at what point do i add freon ? I have tried to add freon with it screwed in half way and pretty much all the way in but it doesn't seem to draw the freon out of the jug............what gives? I have the unit running but can'tseem to comeup with a working combination.