Ac not cool in 420 sel 1989

[janepabst]

A year ago I had to put more freon in my MB. This March I had to put more in. Now the Ac is not cool . The last time a man had a testing device that he put all over my ac system (under the hood). He found no leaks. Should I have the system flushed? I have read -1. no vacuum to water valve-could be missing activator. 2.be sure fresh air intake is completely closing. 3. blend flap in dash not closing of the heater. 4. Replace the receiver/dryer. 5. Get a new thermostatic expansion value. I also found a website that tells you how to vchanf from the old kind of freon to the new kind of freon. Does anyone have any suggestions or solutions? Thanks