BMW Repair: A/C Evaporator (valve and core), inflatable mattress, valve job

Question
I took my 1999 540i sport in two weeks ago as the A/C was not working. They told me the Evaporator valve was faulty and charged me $600 to fix (including system test, drain and replace coolant). Two days after I picked the car up, the a/c stopped working again. You can actually hear the same noise internally coming through the vents (sounds like the air being released out of an inflatable mattress when you hit the a/c button while fan already running). Anyway, I returned the car to dealer who now claims the evaporator core is leaking. My question is 1) what are the odds that I was charged to replace a valve that was actually in good working order? 2) what is the incremental labour (hours) to replace an evaporator core vs. replacing the evaporator valve? I am happy to pay to have it fixed, but I sense that they missed something initially and want me to foot the bill. Many thanks.

Answer
Matt, the chances are very good that they missed the evaporator leak. The Expansion Valve job I would have sold for 5 to 7 hours(with Diagnosis) from start to finish. Now the Evaporator job requires that the whole dash come out of the car. Labor on the evaporator looks like 18-20 hours all included (No diagnosis). My price is based on warranty rate multiplied by 1.75 (tipical for customer pay work) I would talk to the service manager and see if he can help you with parts and labor pricing. also make sure that it is the evaporator leaking and not something else. If the A/C leaked out in two days I would think you would have smelled it in the cabin because two days is a moderate/heavy leak. See if there is some way you can create and adapter out of your old expansion valve (if you or they sill have it so you can pressure test the Evaporator before replacing it. Or make sure that they can show you direct evidence of the leak from the Evaporator core. I can count on one hand the number of Evaporators I have seen fail. More Common is the counter part the Condenser which sits in front of the Radiator. It is subjected to high heat and has road debris thrown at it all day. Compressor leaks are common too. Did they inject UV dye in the system and look for stain or did they use a Refridgerant sniffer to find the leak?
-Evan