If you just had this system worked on, take it back to the same mechanic and give him a chance to fix it. As others have said, the cause of this problem could be one of many, but you need to put a set of gauges on the system to diagnose it. If it is just a matter of getting the system pressurized properly, he should not charge you for this.

Jim
'88 735ia
The A/C on my 89 750 still doesn't work. this time when I push the button for the compressor the compressor doesn't come on. Last time before I had it worked on the compressor came on and the system was out of freon or atleast very low on it I think it took 5 pounds of it. Here's what my mechanic wrote on the work order. A/C presure low-28psi Hi-275psi it had a leak in the expansion valve so he replaced that and the o-rings he checked for other leaks and found none also he cleaned the evaporator core. Anyone have any Ideas? Also the Aux fan does come on even though the compressor doesn't