Go to a parts house, buy a pocket thermometer and insert it into the dash vent. Turn the a/c to max or recirc mode, blower on hi and see what temperature is actually exiting the vents. If the air temp is 35 to 50 degrees then you are likely getting all you will get. It is possible the hot water cut off valve is malfunctioning and allowinghot water to run thru the heater core which will over vome the a/c on hot days. It is also possible the vac operated "blend" doors are no longer working properly and you need that corrected. I have given you the most common problems to check out herein. BTW: 1999 GM and all other makes do not use freon, it is banned- since 1993. You have refrigerant R134A. Your system is not obsolete, it is not blowing thru the refrigerant. Here is a crash course: The compressor pumps liquid R134A under hi pressure from the compressor to the condensor core where it surrenders absorbed heat. Them it is pumped under hi pressure thru an accumulator or drier, then thru an orifice or expansion valve or some type of restrictor into the evaporator core where it is now a lower pressure gas where it absorbs heat from the cabin air. From there is flows under low pressure back to the compressor and recycles over and over until shut down. See if your car has an interior air filter, if yes ck or change it, check the temp at the vents as previously described herein, and be sure the belts are not slipping on the compressor. On really hot days again use max or recirc settings as it uses strictly interior air which is easier to cool than the fresh outside air which is hot. There will be a noticable difference in the 2 settings alone. BTW: the fat line is the low pressure line the dmaller line the high pressure. If you determine that you need a charge of refrigerant and lack the skills, you can be severely injured ot killed if improperly handled or done. Consider your skill levels and act accordingly, saving a few dollars in labor is not worth a permanent of fatal injury.