Pontiac Repair: Pontiac Sunfire AC problem, pontiac sunfire, pressure relief valve

Question
I have a 2001 Pontiac Sunfire.
I have had it in twice now to have the AC serviced.
The first time they found a leak in a line, replaced it and recharged it. Worked for a couple of days. Second time, the exact same thing happened again.
We are at the time in New England where you use the heat in the morning and the AC in the afternoon. It appears to me that when I use the heat in the morning, the AC will no longer function till its been recharged. If I don't use the heat, the AC will remain working. As soon as I put the heat on, the AC loses its charge. Is there a component that could go bad that would cause the AC to dump its gas if the heat is turned on? I was thinking their may be a pressure relief valve or something. Thank you very much for your time and I would greatly appreciate any help you can give.
Regards.

Answer
No, the heat should in no way affect the AC. The only thing that I can think of that may be causing your problem is a temp door that is binding. This is the door that is controled by the temp knob on the heater/AC control head. All it does is block the air flow through the heater core when the AC is on. So if the AC works normal until you change the temp setting to hot, and then the door gets stuck in the heat position, when you go back to AC, you are still geting air flow through the heater core making it act like the AC is not working.
I hope this makes sense to you.
I'm just guessing, but your leak in the first place was probably the evaporator core, right? When you replace the evap core it is very easy to get the temp door in the wrong position causing the problem that you described.