For this evaluation the customer system had stopped cooling, the compressor was running as well as the fan and indoor unit. What I gauged up the customer had a 50-plus degree superheat with a piston metering device and a Target superheat up between 26 and 30. Before gauging up I noticed the Schrader on the saturation line was loose and I could see visible bubbling indicating the refrigerant was escaping from that Schrader. Just to be safe I went ahead and leave search the evaporator coil as well it was not able to find a leak which leads me to believe that the cause of low refrigerant was in fact from the loose Schrader valve.