[GMCNUT]

Originally Posted by BobBray

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.

Apologies for hijacking the thread but I have to ask Bob a very closely related question right quick while he's on this topic - Bob, since you know a lot about how these engines breathe, I think you can answer this question: the 305A engines have a road draft tube and breather caps in BOTH valve covers - if someone wanted to add the 305E type valve covers to a 305A (Plaid covers for example) then doing this would eliminate one of the two stock breathers because the 305E only used a breather cap on one of the valve covers - not both like the 305A. So my question to you is, would the 305A still "breathe" OK with only one valve cover having a breather cap?