Brought my girlfriend's Minivan in for A/C leaking...

Brought my girlfriend's Minivan in for A/C leaking it's 134a charge. The system has been leaking gas over 6 times with the frequency being shorter each time. Brought it in to this dealership and explained fully the problem and they said they would fix it. After a bill you would expect from a dealership and the minivan repaired they said they charged the system but could not find the leak? REALLY? Then they said if it leaks down they will charge $140.00 to look at it again! Well the 134a lasted three weeks! Anyone can go to a parts store and get the 134a and charge their system for a few dollars. And it looks like they do that too! So if you have A/C problems take it to a place that can fix it the first time and stand behind their work. I am very disappointed in their business practices and work ethic.