when you shut down the motor, the compressor "passes" the 200ish pressure to the low side, to equalize the pressure. totally normal.
switching back to R12 requires flushing all the 134 oil out of the lines, replaceing the TXV and the reciever as well as the compressor. 12 doesn't like the ester oil that 134 conversions use.
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!