I have looked through the many of the A/C threads and post but haven't quite found the answer to this one - my appologies if it has

I have a 1983 300SD with a bad R4 compressor . I check it with UV dye and it seems to be leaking in the back where the back plate piece joins the housing . The system works when there is enough refrigerant in the system
Is this a big o-ring seal which is easily replaceable . A friend has told me that the R4 is not s very good compressor . I don't mind replacing the compressor with another R4 if I can find quality rebuilt for around $ 100 . What GM vehicle applications fit - I saw something about 9 o'clock for my caqr but saw them with other coil clock positions ( 2, 12 ) for other applications) . However, a friend has told me that the R4 compressors are not good and it would be better to go back with something else if posssible. What other will fit most easily ( what mods are required ).

Also, where is the expansion valve for this 1983 300SD system located ? Is it in the same location and R&R the same as DieselGiant's description for the 300D ?

Thanks