Just adding information for your consideration.
I suspect the GMC valve cover has an oil deflector inside (also used on V8 engines).
Oil slinging off the rockers can be directed at your vacuum hose. No deflector on the GMC valve cover. I found such a cover and mounted it on my '52 GMC 228. I routed a hose thru a small air compressor oil separator and then into the air cleaner. Every 2 months I open up the separator and pour out about 1/2 teaspoon of oil.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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