Upon arrival, I found the evaporator coil leaking from the valve inside of the evaporator coil used for TXVs. I removed the cap and seen the core was loose. I tightened to core and replaced the cap with a flare cap. I charged the unit with 2 pounds of R410a. The system is now cooling. I am concerned about what caused the leak to occur and the potential of none condensables being in the system now. The system has a piston. It is now cooling with a good supply temperature and differential. I provided the customer with our maintenance brochure. The customer paid for the service and the H/W paid for the dispatch in full. It rained before I arrived and a little during.