I had the same problem two months ago when the compressor in my '91 took a dump.

If you do the work yourself, you should flush the system...problem is a bad compressor sends shrapnel everywhere. If you don't flush it then you risk destroying the new compressor.

I had the 134 conversion...134 sux on the '91...thermal properties of 134 are different from 12. The AC seems to function at half-capacity...takes forever to get cool.