Hi all;
I have a 12000btu airconditioner, when I start the unit on cooling the compressor hums then stops, I have checked run start & common, they are all fine, I have changed the start & run capacitors but still the compressor will not kick in, I am trying to regas the unit,
So I must keep it on cooling to be able to suck in the R22.
Please help!

Is this a window or wall unit or is it a small central A/C system? Put a Amprobe on the compressor lead and check the current draw to see if it is drawing too much current. If the unit was out of refrigerant (freon as you call it) you will have to fix the leak, add a liquid line dryer, and pull a vacuum before charging it. You should weigh in the charge if it is a window or wall unit and then set the superheat or subcooling for accuracy. Many times a small window or wall unit can be charged by just weighing in the charge if the refrigerant lines have not been changed much by the repair but if you add a dryer the weigh in as per the specs of the unit will not work. A/C units do not run out of gas. The gas is low because of a leak.

Now you might have a bad compressor that started all of this fun but without a Amprobe you have no way of testing the motor under load. The run,start windings can be good but the piston,con rod,valves could be having a problem. If the compressor hums without pumping you have a locked rotor situation and probably a bad compressor. I am not there to professionally test it so I have to go by the info you have given.