If it's still cold after six months, it's not a large leak.

Only problem with your proposed solution is the a/c system contains both refrigerant and oil. The oil lubricates the compressor. Both refrigerant and oil leak out of the system. When you run low on oil, the compressor will seize and fail. It's ugly.

As an alternative you could add oil each time the system is recharged. Only problem is you don't really know how much oil to add.

FWIW, a 30lb container of R134a can be had for ~$100. Another hundred or so for guages and such, and you could keep it topped up for a very long time for about $200 bucks. I actually did this for a few years with my 124 until I got around to changing the evap. In my case it was only going about three weeks between recharges; at that point the inconvenience starts to factor in and override the cost of replacing the evap.

- JimY