Here’s what I have on the 72 c30 after I misplaced the one on it. Per advance auto computer. The old one never built up pressure in the tank. The new one built up pressure in the tank. I took a 1/8 drill bit and ran it down the 5 holes on the new cap solved the pressure problem.
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1972 Chevy c30 Flatbed
"Weekly driver as I work on it"
Follow along in the DITY

1938 Chevrolet 1.5 Ton
"On The Hunt For A BIG BOLT"
Found! And the story continues in his DITY bay