yes the evaporator can be a leak point so can the expansion valve.
tell the mechanic to hold abt 150 psi pressure in it - ideally nitrogen, but as its Pakistan he would not even know where to buy the tank and regulator, so just agree to the air check.
the lines can be flushed out with CTC or acetone (not the rubber ones) - dont flush any manifold or muffler either - replace all O rings with new parts and pull a strong vacuum with a dedicated vacuum pump, not fridge ka compressor.
And instead of surf ka pani - stop the little thayla that sells bubbles and balloons to kids and buy a litre of the solution they sell. Its better for leak check - I used that too in Karachi.