if your car is 10 years old, many places on a/c could be leakin

the primary cause is the seal between rotor and housing in compressor. additionally, evaporator connectors, drier connectors and schlodar valves could contribute to the leak too.  the last one is the rubber hoses, they are getting hardened and any micro cracks could cause leaking.  if it's too old, you might need to replace the whole thing for good.