Air Conditioning problem
1991 GMC C1500 V8 Two Wheel Drive Automatic
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NEED TO WHICH SERVICE VALVE IS THE HIGH SIDE
AND WHICH IS THE LOW SIDE

High side is on the smaller diameter line, the low is on the accumulator, you need an adapter to check the high side, Hose set for the manifold gauges will not fit on the high side without it...You may have trouble finding R-12, it's not sold thru leagal channels anymore!