the question is, why is the system low on gas?

they do leak a certain amount over time, but it shouldn't be that much. no point just filling it again, find the leak first. also, how much oil have you lost? does that need topping up too? a professional company will see you right on these points, a bodger will just vacuum the system down and then fill it with gas. be careful with who you use, but one that's recomended should be good.