Before you spend more time and money on R134 cans, you need to fix the leak. An A/C is a closed system. It does not consume R134A so if you are empty, you got a leak.
First check all the plumbing joints where there is a nut or connector for corrossion. Then check the O rings, inside the connectors. If it appears brittle change it.
Next get a can of A/C sealer with red dye. Interdynamics makes it. You will need to jump the low pressure cut off switch, or power the A/C compressor with a jumper wire to get it started, so that it can suck in the sealer. Hook up the sealler the same way you hooked up the R134A, on the suction (low pressure side). After the sealer is sucked in, check the plumbing again for leaks. Usually the red dye can be spotted with a UV light. If you don't have a UV light, get a UV bulb at electric store and hook it up to your shop lite to see the red stuff. It will appear as oily, so you can feel it too.
Assuming no leaks, nows Recharge with R134A. Again jump the compressor so that it can suck. Hold the first can upside down so that liquid R134A is sucked in first. Your car may need 2 cans if it is completely empty.