So here's the scoop. I have a '75 GMC with a 250. It originally had a uni-head, but after it blew the head gasket, I ditched it in favor of a rebuilt '69 250 head. I began daily driving it again and it performed beautifully...

Until I drove it on the freeway for the first time. I drove 30 miles and by the time I got to my destination, crankcase pressure had forced oil out of the valve cover gasket. Not enough to cause a panic, but enough to be concerned. My cheap, chrome, aftermarket valve cover was running a breather in front, connected to the air cleaner and a pcv valve at the rear, connected to a port on the center runner of the intake. My Dad suggested that I remove the pcv and replace it with another breather, so I did that yesterday.

This morning I drove the truck the 30 miles into work, only to find that the pressure build-up had forced almost every bit of oil up and out of the aft (and newly placed) breather. Any ideas as to what may be causing this excess crankcase pressure?? Did I have the pcv connected to the wrong place when I was still running it? Any help will be greatly appreciated, please and thank you!

Here's the current set-up:


The black vacuum cap on the intake's center runner is where the pcv HAD been connected, going back to the aft portion of the valve cover:


Also, when the uni-head's gasket gave up the ghost, I immediately bought a new head gasket: Fel Pro part # FPG 8501PT1. Which is supposedly specific to the uni-head. Fel Pro part # FPG 8006PT is for non-integrated head's. When we installed the head gasket, we checked it against the block and everything looked copasetic. But we didn't check it against the head... Could this be the problem? I reasoned that because the blocks are identical castings, it ought to be ok... Did my assumption spray oil in my face?

Is there baffles installed inside the valve cover for either the fill hole or the PCV area?

#1 cause of crankcase pressure: ring leakage.

Yes. There are baffles on both ends.

Two things: Wouldn't bad rings cause the vehicle to smoke? Because this truck never does. And this pressure issue did not present itself until after the rebuilt head was installed.

I got a good one for ya...When I bought my '68 Panel, I had to go to Phoenix and trailer it back to Utah. Long story short, tow vehicle broke down and I ended up driving the Panel the last 300 miles. I didn't know it at the time, but it had a 230 in it. And it ran just fine. As much power as you would expect for such a small motor in such a heavy truck. No smoke, didn't use any oil, in fact, I ended up driving to work for the next week while my company truck was being fixed. Only problem was TONS of blow-by. It had one of those rubber push-in type oil filler caps, and it would blow that right off when I hit about 50 mph. Scared the hell outta me the first time, it hit the bottom of the hood so hard. Anyway, when I finally got it out and took the head off, everything looked normal, but when I got the pistons out, there was a tri-angle shaped piece of piston missing from #1, with one point of the triangle right where the little notch in the top of the piston that points forward is. triangle was about 3/4" on a side, with the bottom of the triangle about where the oil ring was. The 2 compression rings were broken, but mostly still there.
Funny thing was, there was no sign of any of the broken pieces to be found.

engine needs O/H

When your gasket blew did you get water in your oil? If you did you could have a broke compression ring and that does not mean it will smoke (oil ring still intact). When a gasket blows and water gets in the cylinder it can break the rings. Check your compression. You probably have low compression on the cylinder where the gasket blew. Water can not compress in a cylinder and will cause connecting rods to bend or rings to break. Can also crack a piston but rare.

Many of the after market chrome covers Had a baffle that had a hole in the side of them.( right where you have that rubber plug.And if the Plug did not go into the baffle It would suck the oil in from that side.I had one that did just that.And the breather would leak oil all over the cover. What I did to fix that was a deeper plug. and no more leaky oil issues from the breather. Maybe?????????? that is the same issue for you???
just a 2cent thought.

JUST MAYBE you have the wrong PCV valve, or it is plumbed in the wrong direction. Blow through both directions to determine the flow direction. If installed backwards, you have virtually "sealed" the crankcase with no good place for the "blow-by" to exhaust.

!964 195 hp 283 chevys and 1964 300 hp 327 chevys have similar looking pcv valves, each having threads on one end. One car has a threaded intake manifold, the other has a threaded carb base, they must have two different direction flowing PCVs.

Check it out --its cheap

Whoops, I did'nt read completely-- the above dont explain operation WITHOUT THE VALVE. Maybe there was damage to a piston...when you have water in a cylinder and you hit the starter, it sounds just like your timing is real high and you have a weak battery-it will stop your starter (cant compress liquid). Usually the was cylinder wall will crack, but it could damage the head gasket or piston, or anything sealing the cylinder. A cracked cylinder wall would not expose crankcase pressures, a broken piston could... do you have milky colored oil...which could be block/head crack or head gasket failure and some weird way provide opening to the crankcase? The VOLUME of oil makes me think of a damaged piston.