98 Deville A/C problems, No leak???

I am having trouble with my A/C. I had a friend hook up his gauges and there is plenty freon. Also he used a device that detects leaks and we didn't find any..I still keep getting the "low refrigerant - A/C off" message. It automatically cuts of the compressor. We checked the low pressure sensor, but it had continuity so I really don't know what it could be. Can someone please help, hot summers are coming to Texas and I can't drive my 5 month old son around in this heat.....