98 540i a/c problem

[hollywood1178]

hey guys...
took 540i in to get inspection, upon picking it up was told that during the inspection the a/c blew all the freon. i attempted a new charge on the system and after 1 can, then 2, no leak/no cold air either....put half of a third can in and then the a/c started BLOWING out of the back of the compressor...
is this a pressure relief valve?? sounds to me as if I might have a blockage in the system preventing it from cooling and circulating resulting in a pressure release...and IS that a pressure relief valve on the back of the compressor??

[Geschwindigkeitswagen]

I haven't looked at the BMW compressor yet, but I've always had my system in my other car vacuumed down before trying to charge it. Do you do that first? If not, even though there was no refrigerant in the system, it is under pressure and adding more coolant may have over pressurized it.