Im helping out someone at work and her AC was filled and checked for a leak. no leak found and now (3 months later) i hooked up the pressure gauges to the ac and the low side had pressure but the high side had none. when I took the cap off the schredder valve on the high side there was freon around the valve. When I turned on the AC the Compressor does not engauge. Should it engauge even without freon? is the freon on the valve a good indication that maybe the valve needs to be replaced. using the dye and sniffer or uv light won't work, the guys before couldn't find it, prob because it is a slow leak. any ideas?