I'm having the same problem in my 67 1/2 ton. I have one of those aftermarket chrome ones which vents from the bottom. I do alot of hwy miles and find lots of oil spray all over the valve cover. Here's the kicker, it had the original oil fill cover, but on the recommendation of the guy who replaced my rear main seal-it was replaced. The wierd part is that my Camaro is the exact same design and it regularly sees high rpms and I've never had a problem blowing out the rear main seal.
I too, am wondering if there's a breather that won't blow oil out the bottom.
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67 Chevy truck 250(no, I'm not putting in a 350!)
68 Camaro 250(again, I'm not putting in a 350!)
