2002 Mazda Protege Freon always blow out

I had AC problem ever after I asked the shop to check my AC system. It worked fine before the check, not very well, but fine.
My mechanic told me AC compressor is weak and needs to be replaced after his check. Then I replaced the AC compressor, new one. It seemed working after the replacement. However, AC failed by blowing out freon from the engine part or bottom of engine, which one I am not sure, after I went home the same day. I took my car to the mechanic the second day and they found out the fuse has some kind of problem. My mechanic checked the fuse in engine part and in car chamber. After fixing the fuse, AC worked again. But the second day, freon blew out again from the engine part or under the engine. I took my car to the mechanic again and he told me to replace the AC accumulator/drier. Then I replaced it. It worked again by then. But before I drove out of the shop, Freon blew out again from the engine part or under the engine. My mechanic was surprised and he did a more thorough check this time. And then he found AC expansion valve is down. Then I replaced AC expansion valve according to his advice. It seems working finally now, and very well. I thought it will be ok after then, but today Freon is blowing out again from the engine part or under the bottom of engine. Each time when freon blows out, it only happens when the car is idle. When I drive, it never happened.
Every time when my mechanic repaired or replaced any parts, I was present all the time and watched him work. He told me there's no sign of leaking when he vacuumizes the AC system. And I saw the pressure gauge when he did vaccumization, there's no pressure drop at all. I assume this means AC systems's sealability is good. But where's the problem?