A/C R134 retrofit from prev. R12 = poor performance..any ideas?

[JordanVw at aol.com]

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i had R12 in my a/c system up until a few weeks ago, then it stopped blowing
cold. so i was advised to retrofit my '86 5kT to R134. Now, with the R134
retrofit, the performance sucks.  its not even enough to cool the car.  with
the R12 it was nice and cold, now it's not.     mechanic says that's all i
can expect from a retrofit.  is this true??
 any ideas?
TIA
chris
'86 5kT w/ lousy a/c