I just rebuilt my 350 not to long ago and I made sure I would never have any leaks. Therefor I put a little more than enough silicone on the front and rear oil pan seals, timing cover gasket, and intake front and back valleys.
I never really thought that much about the breathers and pcv valve except that I needed a pcv valve that was functioning.
THe one breather on the other valve cover was a bit old and the coils supporting its structure were squished a bit.
Until a couple weeks ago I started to get some major oil leaks and was I pissed off. I put so much silicone and made sure all the gaskets were seated properly now everytime I would park somewhere I would look underneath and there would be a small puddle of oil that dripped from everywhere possible including the oil pan.
NOT only were the leaks a problem. I was on the highway and guess what ...my oil dipstick popped out, my pcv valve popped out, my breather popped out. So I spent 1 hour of my day and bought a new breather and put washers on my baffle(old Mickey THompson valve covers have a screw held baffle that you can adjust) so i could slide the pcv valve fully in.

I went from leaking insane amounts of oil to NOTHING.
I hope this post will help remind people like myself that if you over look crank case breathing,, your new high end rebuild could turn ugly( you could blow all of your oil related seals)...
Good luck and take care of your internal engine breathing:hail:

 

I know what you mean. That's very true. If you build ANY meaningful pressure in the crankcase it WILL find a path out.

Just 2 weeks ago on my wife's 92 Camaro I was having some problems. It's got a fresh carbureted 350 and I was very upset to find oil vapors drifting slowly out of the breather on the valve cover at idle. I run a PCV valve on the other cover so I started to think maybe the rings didn't seat up (lots of blow-by). Skipping to the end.... I eventually discovered that the brand new PCV valve I bought was defective right out of the box!

When the PCV valve is exposed to vacuum from the carb it retracts a small needle valve against it's seat inside the valve. It should still be able to pass a considerable amount of air past the needle valve even when fully retracted. Mine was sealing up tight as a drum. No vacuum leak at all. So... the crankcase blow-by gasses were finding the easiest path out of the engine- the breather on the other valve cover.

HOW TO TEST A PCV VALVE FOR THIS PROBLEM: Use a small screw driver to force the needle valve in the PCV onto it's seat. Do this by pushing up on the metal flapper on the bottom of the valve until it bottoms out. With it fully seated see if you can blow through the vacuum port on the top. You should have some resistance, but it should still allow you to blow a fair bit of air through it. If it doesn't or if you can barely push any air through it at all, it's toast.