Tech replaced an evaporator coil under manufacture warranty, tech did not recover charge due to the system being with only 30psi(no refrigerant in the system). Tech replaced the filter dryer and evaporator coil while flowing nitrogen. Tech pressure tested to 300psi and noticed the pressure dropped 40 psi within 20 minutes, tech then pressurized then system to 350psi and leaked searched the joints of the indoor(new coil brazed) and outdoor system and did not spot any, as tech walked to to the outdoor unit to check the filter dryer, dirt and mud blew from the PVC chase and noticed that the system lost all the nitrogen pressure, tech attempted to remove as much dirt as possible to attempt to sport the leak (nitrogen was flowing and headed water gargling) tech went inside and and removed spray foam insulation to attempt to push the liquid line thru to see if the leak is near the outside and did not move much, tech suspects the leak in the line set being somewhere under the slab where it is