The factory R12 A/C system on my 1988 Mazda B2200 has had a slow leak
for the last 10 years.  Normally adding one small can of R12 in the
Spring would get me through until the next year.  Last year the small
leak apparently became a major one: the sight glass looks like all the
freon is gone and the A/C doesn't cool at all.  The compressor kicks
in normally.

Given the high cost and scarcity of R12 (I'm down to my last 3 small
cans) I'm wondering if I'd be better off having a professional A/C
shop convert the system over to R134 after they locate and fix the
source of the leak.  An article which came out in Family Handyman or
some such do-it-yourself magazine back in the 90s listed the B2200 as
one of the more expensive models to convert, because major components
have to be replaced along with the O-rings.  However,  people lately
have told me it's not as big a deal as it was once made out to be, at
least for most makes/models, and on most cars the R12 components will
work with R134.

Can any of you refrig techs or fellow B2200 owners give me the
straight story, and tell me approximately how much the conversion job
would cost assuming the leak is just a bad O-ring or seal?