The sight glass isn't an effective indicator with 134a, only works with R-12.
Yes, I think you should use R-12, the system was designed for it and it is more
effecient than 134a. To go back to R-12, you have to flush everything but the
compressor, replace the accumulator,run mineral oil thru the compressor to
flush the other oil and recharge to spec with R-12 and proper amt. mineral oil.
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