I work on auto ac a lot and you are working with a low charge. You need to find out where its leaking. A good way thats free is to look for oil. If you have a leak you will see and or feel oil where the leak is. I have to say that 90% of any leaks I find are at one of the two valve stems. If you take the caps off listen real close for air leaking out. If you hear a lil air coming out or see oil then thats the leak. Easy and cheap fix. You need to buy a can of r134a with the gauge already on it. Its too hard to screw anything up with the gauge that says where to stop charging. Hope this helps.