Re: 1967 gmc
Should be O.K.. The PCV valves are under the valve covers, so you have to allow fresh air into the crankcase. Most V-6's have the can with the small filter in it behind the left head. On California 'closed' systems, the can was replaced with a pipe to the air cleaner. Later on (about '68 and up) the hole where the can or pipe attached was blocked off with a freeze plug and a breather was added to one of the valve covers, usually with a hose to the air cleaner. The filter in the can, pipe to the air cleaner, and breather on the valve cover all do the same thing: they let air into the crankcase.
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