I've been using 406-a in my old accord for years now. I'ts about as "drop
in" as you can get. I've found that 134 performance in older r12 systems
leaves a lot to be desired. The 406-a stuff is dirt cheap, and my vent
temp on a hot summer day is under 40F. Its also compatible with the R-12
oil, so you don't have to worry about getting all the oil out. Do a google
search, you'll find it. You'll also need to take an online EPA test to
order it, but its really simple.
  Barring that, the proper way to do a conversion is to drain the system,
change the dryer and all the orings. It can be a huge pain in the a.s and
often other problems can pop up when you get into 15+ year old cars. But,
I've also seen people get reasonable performance just by dropping in 134.
How long that will function, you can't say. I'd still recommend the 406-a.

You might want to link into the forum associated with www.ackits.com

You can get really good professional information, as well as any parts you
may need,
via this site.

It is my opinion that, if you want a cold car, you dont want to fiddle
around with
half baked conversions.  Do it right.

And that means replacing
- the o-rings,
-washing out the system,
-replacing the  lubricant in the compressor with compatible lubricant,
- replacing the filter/drier,
- PERHAPS replacing the expansion control be it valve or orifice type,
-pulling a good vacuum and
-properly recharging with 134a.

Anything much less is jackleg work.

Here is the response I posted on the R134 conversion to another
question some while ago. BTW, the Interdynamics conversion video says
that many shops will recover R12 for free because it's like cash in
their pockets.

AC would still cool a little but not as much if there is less freon. Do
you see bubbles in the receiver/dryer sight glass? If so that's
probably it. Systems are considered "sealed" even if they leak 1/2 to 1
oz of freon per year.

If you see visible leaks along the metal freon lines to and from the
compressor, then you have to think about whether or not the o-rings
should be replaced. Otherwise new freon will leak out of there. O-ring
conditions in aftermarket recharge kits may help if the o-ring is not
in too bad of a shape.

Also check the operation of your AC condenser fan motor. Do this by
turning the key to ON without starting the engine, then turn on the AC
(only the blower motor will turn of course). Beware of moving parts!
The condenser fan in the engine compartment should operate. You should
also check drive belt tension. It's harder to check the belt tension if
you don't have a gauge, but at least the belt that drives the
compressor should not be loose. Typically you should have about 1/4"
deflection at mid point, and on farther spaced pulleys 1/2". It could
also be a problem with the compressor or the clutch slipping. But if
you don't hear abnormal noises then they are probably OK.

I don't know if you can still find R12, but Interdynamics has a R12 ->
R134a retrofit kit. It's probably cheaper too.

http://www.id-usa.com/product.asp?CID=28&PID=217

But you still have to have the system properly discharged, pulled full
vacuum for 30 minutes, and then recharge with the kit without
introducing any air (air forms a combustable mixture with R134a).

If the only problem is lost freon and all other parts are in acceptable
condition then buy the kit ($60) and have a shop pull vacuum for you.
You then fill it with the retrofit kit. My guess is no more than $100?

Check out the how-to guide on the Interdynamics web site

http://www.id-usa.com/how_to_videos.asp