A '93 is an R12 system. A retro-fit kit to use R134a (or equivalent)
requires:

a) Remove the compressor, drain the old oil and replace with r143a
compatible, swap the H and L pressure valve fittings.
b) replace the drier cannister.
c) replace ALL the O rings wth with r143a compatible rings.
d) Evacuate the system and see if it holds a vacuum. If it does, recharge
with R134a (or equivalent). Note that you cannot just charge up a system
without evacuating it first: it won't properly or for long.

Without the right equipment (air driven vacuum pump and gauge set) you can't
do d) yourself.

You could do a,b,c and then take it to an AC shop and have the vacuum test
done. They will likely want to charge you $200 or so to tell you that there
is a leak somewhere.

Note AC shops have to abide by a whole lot of government regulations, which
is why they are often outrageously expensive.

For a 93, the most common leak source is the condensor  in front of the
radiator. This has taken 15 years of road dirt impacts and winter salt.  The
next likely source of leaks is the hoses themselves. With age and the change
of refrigerant, the smaller r134a molecules leak out through the walls of
the hoses themselves. The only solution is to have the hoses rebuilt (don't
think about buying new ones, $$$$$).  I had some done for my 93 last year.
Ask around your local A/C shops: many won't do it, but there's usually
someone who will. The process requires that new barbed hose fittings be
welded onto the existing pipes as the mitsu fittings are odd. The hoses are
then crimped onto the pipes.

You might consider the following:

1. Get a new condensor: you probably need one anyway, also the drier
cannister. Look on, say, RockAuto.com. Before you order one, pull out the
compressor and check it out. You should be able to turn the shaft and feel
the pressure on the in/out ports. There should be no (black) oil leaking out
the shaft. If the compressor seems faulty, consider buying a new one. I have
a decent 93 compressor here in Ottawa, Canada: its yours for the shipping if
you want it.
2. Get the hoses rebuilt. Can't hurt and is cheaper than new.
3. Buy an air driven vacuum pump and gauge set. It's about $200 or so. You
hook the pump to a compressed air source and away it goes.  This is a good
option as its about the cost of having the system leak tested once.
Rebuilding an old system often takes several goes at finding the leak. The
simplest test is vac out the system. The vacuum should hold with no change
for at least a couple of days. If there is a leak, its usually pretty clear
immediately as the vacuum leaks away in seconds to hours. Hence my
suggestion of replacing the condensor.

You can also vac out the system yourself and put in an R134a equivalent.

regards
Stewart DIBBS
www.lancerproject.ca