Thanks to all who participate. Just a quick note that may help someone trying to trouble shoot their ac system. Symptoms: System would come on and work normally until car or ac was shut down, re-engagement was not possible. (i.e. warm under hood)
Solution: Replaced the AC compressor relay. Problem SOLVED

Guys and girls, I went through the whole drill checking everything,flushing evaporator, replacing accumulator and orifice tube, evacuating and refilling, testing with gauges etc.

1991 Camaro converted to 134a, v-8 with March pulley system and Sanden compressor.
The relay is a $10 part, cost me @ $200 to figure it out, but I now have an AC gauge set and vacuum pump, new orifice tube and freon! OUCH!