A/C recharge from empty

I looked through a load of threads but only found those that needed a small recharge, not full.



I while back I replaced my heater core which meant draining the entire A/C system. If I know the whole system is 24oz can I just buy 24oz worth of R134 and put it in there? I keep reading about having to vacuum the lines of air THEN put the refrigerant in which means $200+ at my shop instead of $30 for the cans of A/C. But of course I don't want to completely destroy my A/C system.



Thanks for any help!