While not a pro, I've been around the block a few times on AC systems.
First - get rid of the miracle-in-a-can AC recharge/Leak fix/lose weight while you sleep products. They can often cause real harm. Here's why. An AC system consists of a number of elements, but one could argue that what really makes it all work is the restriction or orifice. This is a tiny orifice that creates a back pressure in the system for the compressor to work against. By forcing compressed liquid refrigerant through this orifice into the evaporator, it causes the refrigerant to expand and vaporize and in the process, absorb a bunch of heat. This is what makes it all work. If you put in something meant to plug a small leak, well guess what it may very well end up plugging up?
There is only one proper way to do this, which is to get a set of gages. They have come down in price and up in availability so much that it is hard to argue against it. Unless you know what is going on on BOTH the high and low pressure sides, you really don't know what is going on. They may look confusing but are quite simple. They are color coded/sized so you can only connect the high pressure hose to the high side, and vice versa on the low. The part that puzzles people sometimes is the valevs/knobs. The LP hose is always connected to the LP gage. The HP hose is always connected to the HP gage. The knob opens the respective side to the CENTER hose, which is where you attach for vacuuming and recharging. Always start with both valves closed. You only add vapor to the LP side, if you want to be safe (can upright). So if you want to play it safe, only open the LP side valve when charging and keep the can upright. Getting the last bits in can be tough as the can loses pressure as it chills. So I get a pan of hot tap water to dip the can in and keep it warm. Plus I shake it a bit. Speeds it up a lot.
Now you stated you had a pressure of zero (assuming you trust the gage on the can...). You cannot just refill a system that is at zero as you will do nothing but mix new refrigerant with air, and make at best a poor performing system. The first step is to evacuate the system with a vacuum pump that is made for the task. These are not so common as gages, but can be found online for not ridiculous sums. Once you have fully evacuated the system, you can then charge it up through the gages with nothing but pure R134a, with no extra secret sauce in it. If you don't want to buy a pump, you need to find one or take it to a shop.
But backing up a step - you have a leak...as you know. If it is a slow leak on an old car, you may just want to keep recharging from time to time as it may not be worth the cost to find and fix the leak. And they can be a real bugger to find. I have chased one on my wife's Exploder for the past 5-6 years since I foolishly had a shop replace the compressor that went bad (didn't have time to do it myself. Grrrr) The car is old, and it needs a shot once a year in spring, so I just have been recharging it for a few years. We have maybe one more year left on this car. In fact, I just did the spring recharge today, but the leak appears to be a bit worse as it was reading zero for the first time. So I evacuated it, waited to see if it held (seemed to) and recharged it. My theory is the seals are shrinking enough in the bitter cold winter to let it leak out. We had an extra nasty winter, so that may be the reason...or it may just be getting worse.
You can try finding leaks with a UV kit. All systems come with UV dye from the factory, so you only need the light and the yellow glasses, and the darkest garage you can find. The glow of the dye is extremely faint, which is what makes it so hard to find. You will want to sit in total darkness for several minutes with the car (to adjust your eyes) before you try looking for it. Plus all the dirt and grime can obscure the dye too, so it pays to do a thorough cleaning first. Another hint - always wrap the gage fittings with paper towels when connecting/disconnecting otherwise you will spray UV dye everywhere from the little bit left in the fittings and then good luck tracking down your leak. I have never been able to find a leak this way, but sometimes you get lucky...
This is one of those things that takes more time to write than to show.... Hope this helps.
