You probably have a leaking evap core. If the pressure was at zero, it will take 2 cans of r134a to get it up to speed.
What I used to do, when it was really warm out, was add r134a until the line coming from the evap got as cold as the line going into the firewall. I had to place the can in hot water on cooler days to use up the whole can.

If you over pressurize, there is a pressure release valve which will suddenly pop and belch out any excess. Quite an unnerving experience!! with green fluid on the ground and the compressor making all kinds of strange noises.

If the system looses pressure after about 2 weeks and you don't see any yellow dye (add a can of it), it is probably the evap core. Which means replacing it as well as the reciever/drier bottle. There are cases where the reciever/drier bottle has preforated and developed leaks, check it by wiping it with a paper towel.
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My name is Klaus and I am a V ♂ lv ♂ holic.