Arrived at customers residence and was led to the thermostat and error code 83 low pressure lockout for four hours. Put gauges on system and found that the system was flat again. Took so bubbles and covered every joint. Could not find leak at this time. After trying to trace leak from the indoor unit and the indoor coil was unable to find leak at this time. After lengthy leak search popping up system to 400 psi under the impression that leak would be heard due to the high psi. Unfortunately that was not the case today. Isolated outdoor unit pump system up with nitrogen to 300 psi. Closed both service valves to isolate outdoor unit from indoor unit to customer would be back in a couple of days to check isolated unit. Left heat pump system with the ability to use emergency heat until problem with system is resolved at this time. Spoke with Ryan Goring about the leak check and he advised me to shut an outdoor unit from indoor unit to isolate leak at this time.