I have had a 270 in my '53 Chevy pickup since the '70s. This has always bee an issue. Once I used a product called the Right Stuff. I sealed it just fine but was a #@%& t get off the next time. Now I have a cork gasket from BEST gasket. I noticed two things #1 it was new #2 it was thicker. I ran a bead of RTV on the cover when I installed it so the gasket would come off with the cover the next time. It works very well. Now it just blows some out the breathers. I have other leak issues that need attention.
