[tbucketnut]

On my 64 305E, I had to reinstall the valve cover breather because I get too many crankcase vapors and it has caused a leak at the valve cover above the starter. I firmly believe this is because my valve guides are worn causing combustion gases to enter under the valve cover, could also be the reason I have a couple lower compression readings in two of the cylinders. I plan on doing a leak down test to confirm. I haven't studied the GMC pcv system yet but if like other systems it is a controlled vacuum leak into the intake runner that is sucking out unburned crankcase vapors. It has a specific orifice to control a specific amount of flow, beyond that, will remain crankcase vapors that can pressurize the crankcase causing leaks like the one in my valve cover. I have heard people having rear main leaks and gone through expensive repairs to replace the rear main only to have the new one leak too, this could because of a broken ring or worn valve guide or seal too.

Also, I see some of the 305 engine series had road draft tubes/ vented valve cover caps, others after 1962, had pcv systems with the filter canister in place where the road draft tube usually resides and non vented valve cover cap and pcvs screwed into each cylinder head's intake port. My truck was made in Oakland Calf. and was probably one of the ones with calf. emissions requiring it to have the pcvs. Haven't looked into the WW carburetor yet to see if jetting was different because of this but suspect there were jetting changes too.