I have found on these systems which use an expansion valve that loud hissing generally does correspond with low refrigerant level. In this circumstance the evaporator is generally colder on one side than the other. You may be able to observe this by checking the temperature of the heater core right & left sensors (#3 & 4).

That said, the 39 degree evaporator temp is spot on, though 6 bar (~90PSI) is quite low for the high side pressure. What was the outside temperature at the time? I'm guessing the a/c wasn't under much load when you read the values.

- JimY