Dodge Caravan A/C

I have a 2001 Dodge Caravan (3.3 litre engine) that has given me fits over a couple of issues.
1. Air conditioning.....last summer I had it "topped up" and it lasted only for two or three weeks. The guy doing the topping up could not find where it was leaking.
Does anyone have a suggestion as to what or where the problem could be?? Does a condenser act that way???????
2. I have had a strange noise in a rear wwheel ever since I had new brake shoes and brake drums installed last Novemeber. When you are braking you often hear a sort of "thwack, thwack, thwak" sort of sound coming from that rear wheel. We replaced the new drum with the old one and the noise seemed to disappear. When we put a NEW new drum back on the noise came back. I can see maybe the first "new drum" being warped or something like that but not TWO new drums.
Any suggestions??
many thanks
Lansdowne

A/C - If you've had several sets of eyes look for the leak without success, you may be unfortunate enough to have a leaking evaporator or pressure valve behind the dash. You have a dye in the refrigerant correct? I've got the same problem except a recharge lasts me about 2 months. I get it vacuumed and recharged at the beginning of summer then when it depletes I service it myself with a couple of cans of R134. Usually good for the rest of summer.

Brakes - Bit fuzzy on details but, I'd suspect the rear brake shoes weren't adjusted correctly when installed. Yes, they are self adjusting but, they have to be set in range for them to self adjust correctly.

Thanks a ot for your response to my A/C and brakes questions. I will respond when I have some information. The A/C problem you refer to sounds expensive to repair.....amd I correct in that assumption?????
thanks again
lansdowne