I had a 98 Lincoln Continental that was leaking 134a. Would refill and it continued to leak. Saw a local A/C person who told me that it was leaking from the evaporator, under the dash, and that it was going to cost me $xxx.xx. I told him that I could not pay that much, so he advised me to go to NAPA, and buy a couple cans of Stop Leak, and that would help. OK, I did just that. At first, the Stop Leak slowed down the leak so I only had to refill once a week. Added more Stop Leak, and then, only had to refill once a month. Added more, and when I sold the car, I had not added any in over a year. STOP LEAK (from NAPA), WORKED FOR ME.
Whenever I've added 134a (or even R12 years ago) I never evacuated the system out, as long as there was still pressure in the system. (If there was pressure in the system, and the compressor would cycle quickly, air did not enter, only 134a leaked out). I would just add, and never had problems.
I always add a "bug" screen in front of the condensor/radiator, as soon as I bring a new car home. Here in Florida, when it is "Love Bug Season" (May and September), I also add an additional screen in front of the grill.