If you do much auto a/c work, it is inevitable that you will have to repair a leak somewhere in an empty system. That means breaking apart the system, replacing the o-ring, the hose, the compressor, the evaporator or whatever is leaking. Any time you OPEN the system you MUST evacuate before recharging. Otherwise the moisture left behind will not only prevent the system from cooling properly but will also combine with the refrigerant to make an acid that will eat its way through aluminum components from the inside out.

Since you don't currently have a situation needing evacuation, take your time and try to find a bargain on a vacuum pump.

Good luck,