Thursday afternoon went to a call with a 40' long open bunker frozen food case that was down. Case and condenser are about 30 years old, running R408 for several years now. I found condenser cycling on head pressure, 1 of 2 fans was out. Replaced the fan motor, restarted, temps pulled down. Only problem, noticed sight glasses, (2), were flashing. Returned Friday around lunchtime and added 12lbs of 408, the receiver holds aprox 30 lbs. Glasses were still flashing, I was out of gas. There was ice on the suction line at the condenser, normal for this system. Store closed Friday nite at 8:30, temps were good. Saturday am, the on call tech called to tell me, during the night, case went down, lost all product. He said it was out on head pressure. He let "a little gas out", restarted system, and had good pressures. He then cleaned condenser coil, no change in pressure. Any ideas what went wrong?