I took my truck in for A/C service and they found a leak in the evaporater core and accumulator. They said $1200 to repair. It takes about 2 months to leak out. Here in Erie I need it about 3-4 months out of the year (about 4 days this year). Is it possible to charge it up with the autozone stuff a few times a year or does the system need pumped to a vacuum?