If there has been no filter in your unit then for sure the evap coil is
dirty. That being said, it does not explain the lack of cooling on the
lineset or the air coming out of registers. You need a competent tech
to come and check the whole system over. Could very well be a leak on
an old system on the other hand techs who just "throw in a pound of
freon" because they don't know any better are not helping the
situation. Also the advice to " vacuum the system down and weigh in the
charge" is useless unless you can find out where the system is leaking.