As it leaks it will cool less and less till no more.

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_retrofit.asp