I've converted quite a few vehicles (a couple GM's and the remainder MB's) from 134 back to 12 with a great deal of success. I have just recently converted a '82 300SD to 134 (it was a wreck rebuild) and went with a parallel flow condenser during the rebuild. It seems to be working fine for right now and the pressures were not really that much higher than 12. I would not convert one without going with the parallel flow condenser though.

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Jim