Check your blue "relase" valve. The threads should be on the inside of the
stem. Not on the outside. The outside should be smooth. It is for a quick
release attachment. Not threads.
If it is smooth, you R in luck. Your system is 134a. From your symptoms,
you are low on 134a refrigerant. A trip to Advance Auto Parts for a can of
134a and a charging hose is in order. About $30.

It needs major work to be converted.Remove and drain the oil from the
compressor. Backflush all the lines(50 ft),Add new oil to the compressor
and lines,Put a vaccum pump on it for a few hours, and fill with 134a.This
is the miniumum to convert it.