the thing to know here, is that if you're charge is low, you have a leak somewhere. if you pull a vaccum, you will pull moisture into the system. you have to find that leak, or be ready to keep adding to your charge. it may work for a while, (depending on the size of the leak). when i leak test a system, i remove the remaining charge, pressure the system to about 250-300 psi. i use an inert gas, sush as carbon dioxide, or dry nitrogen, then go over the sealed system solder joints,with a solution of dishwashing liquid, & water. look for oil , this is often the sign of a leak. if you can find the leak, you have done the half the work, & often the most difficult half. although most leaks are repairable, some are not. i would suggest repairing it, ( if possible.)