I have a 1982 300 TDT.  The AC on it went out about 5 years ago.  The
compressor went bad and I had it replaced.

The owner of the specialty AC shop I use in Las Vegas, Auto Air and
More, will not use rebuilt compressors.  He says that he has had too
many reliability problems with them over the years, and so he only
installs new compressors.  The price difference is not much.  You also
have to install a new cleaner/dryer when you replace the compressor.

I don't know if it was because I changed the compressor and the system
began to operate with greater pressure or if it was just the age of
the car, but after I replaced the compressor the evaporator and
condenser and 4 of the hoses began to develop slow leaks, so I had to
replace those too eventually.  I have spent about $2000 on my AC in
the last couple of years.

At the time, the shop recommended using Freeze-12, which is a
refrigerant that is environmentally better than R-12, but is
compatible with the type of oil, mineral oil, that is used with R-12.
Freeze-12 is a more efficient refrigerant than R-134a, but is still
not quite as good as the old R-12.  R-12 works well enough to keep
cool in Death Valley on a hot day.  I am not confident that my system
would work that well with R-134a.

I live near Death Valley where we see temperatures in the 120 F range
nearly every day from mid June until early September, so AC is
necessary here.

http://www.freeze12.com/

At the web site above you can purchase Freeze-12 in kits that allow
you to recharge the system yourself.   EPA regulations allow anyone to
work with Freeze-12.  You have to have a license to work with R-12, so
Freeze-12 also has the advantage of being able to do it yourself.

If you do change over to R-134a you have to make sure that you purge
the system of the oil that was used with R-12, because R-134a is not
compatible with mineral oil, and it supposedly turns into a gel in
your system if they combine.  I have not tried it myself, but a local
mechanic here told me he put 134a in one of his cars without purging
the system and it worked fine.  But I would recommend Freeze-12
anyway, and so does he.   Like I said, with our weather here AC is a
vital necessity.  You can buy the cans of Freeze-12 at any NAPA shop
around here.

Here is a routine for filling the system.  It sounds mysterious at
first, but read it a few times if necessary, and it will start to make
sense.

You only need about 95% as much Freeze-12 as R-12 because the
molecules are different in size.

Jon E from Prescott, Arizona (337 days 23 hours ago.)
Once your system is leak tight, contains the proper oil to match the
refrigerant to be used, has been evacutated to remove any air and
moisture contained therein, and has a metering device designed for the
refrigerant to be used, then one may charge the system thusly:

1. Attach the refrigerant container to the low pressure (suction) side
of the system.
2. Open the refrigerant container, thereby breaking the vacuum, and
allow as much refrigerant to enter the system as will go into the
static system. Then, shut off the valve to isolate the refrigerant
container.
3. Place a floor fan in front of the car so that air is directed
against the condenser (located in front of or is part of the radiator)
to simulate ram air (the air flow generated by a car's movement).
4. Start the car and adjust engine speed (weight on foot feed?) to
approximate approximately 40 mph.
5. Place an accurate AC type thermometer in an air discharge duct
which has the best air flow.
6. Turn on the AC.
7. Open all the car windows. (Note that the car needs to be in a warm
environment of around 90 degrees f for charging to work best and most
accurately.) This keeps the heat load on the evaporator (cold coil in
the car).
8. Check the thermometer continuously.
9. As refrigerant is added, the temperature will come down for so long
as the evaporator is not completely full of refrigerant.
10. Go easy, adding a little bit of refrigerant and waiting upon the
system to equillibrate.
11. If the temperature drops, add a little more refrigerant and wait
and watch for drop or not.
12. As soon as you notice that the addition of refrigerant has not
caused any temperature drop, you have the system charged to the point
where the evaporator is maximally used. STOP adding refrigerant at
this time.
13. Dismantle your setup and you are ready to drive a car with a
properly charged system.

This works with any refrigerant.

Note that R-12 could be charged as either vapor or liquid, with the
caveat of not slugging the compressor valves with liquid. R-134a and
other azeotropic refrigerants separate in the gaseous state and must
be charged as liquid. Their cans are designed with dip tubes so that
liquid comes out when the can is upright, just opposite of R-12 cans.
Again, don't slug the compressor. That requires metering the liquid
through a small oriface, which are available as individual divices, or
by cracking the container or gauge-set valve and thereby causing the
liquid to flash into vapor before entering the system.

I am not an auto mechanic, but an old retired HVAC tech who
understands the physics and has charged all my autos thusly and those
of my friends. Prior to this method, I tried all the other methods,
such as, sight glass, pressures, and weighing it in. No method worked
better or as well as this method.

As for converting a system, I beleive (as best as I can remember) that
the oil and the meteing device should be changed when going from 12 to
134a. All systems should be vacuumed, although smaller systems used to
be "blown out" with the refrigerant by many techs. All other
considerations aside, that practice absolutely is illegal today.

If a system is empty when you start working on it, it needn't be
recovered, but if any refrigerant is in it it must by law be recovered
and properly disposed of, as must the old oil. Also, for residential
and commercial systems, you by law must hold an EPA certification to
open a system. I believe there is an automotive certification, but I
am not certain if it must be held to work on automotive systems.