I used an electronic leak detector inside the car. About 5 inches from the front of center vent the indicator would go off consistently. You mentioned that it could be the expansion valve. On other cars I've worked on, the expansion valve is fitted to the evaporator unit. If this is the case, wouldn't it be just as hard to get to the expansion valve as the evaporator or are you saying that the expansion valve in a more accessible area? Also, I've heard that this is one of the weak points in these cars. IIRC, Mercedes changed from a copper evaporator design and went to an all aluminium unit in the mid 80's. Any acid that formed in the system would eat away at the aluminum and cause pinhole leaks in the evaporator.