Unknown Oil Leak--1958

[Cadillac Nut]

Hi,
An acquaintance of mine has pristine original 1958 Sedan Deville.  His father purchased it new.  It has always had a rear engine oil leak that no one has been able to figure out, including Cadillac back in the 1950s.  It's not the rear main seal or valve cover gaskets.  Valley pan gasket & oil pressure sensor were changed, no improvement.  Oil runs down the blow by tube at the rear of the engine to the ground.  Only a few drops, but leaves that unwanted oil spot.  I couldn't think of what this could be.  Anybody have any opinions or suggestions? 

Thank you,
Garrett Baker   
PS:  I am posting this on the "general discussion" section as I thought it was more of a topic for general discussion than technical. 

[Glen]

Does the oil run down the inside or outside of the blow by tube?

[Roger Zimmermann]

Excessive blow-by? It would be interesting to measure the cmpressions; maybe one cylinder has a problem...

[Grant Owen]

It could be a leak from a Rear Oil Gallery plug, I have seen similar problems before mainly in rebuilt engines .
I have also seen a similar leak in a low milage 64 that no one could figure out where the leak came from it turned out to be a crack by the Gallery plug. The only way to properly check for a leak here is to remove the trans let the engine run build up oil pressure & check for leaks . You should also note that if it is a leak from the rear the Flex plate can pick up the oil throw it around so you might see a drip where you wouldn't expect . This is only a suggestion of what the leak may be.

[Dave Shepherd]

I would clean up the area best possible, get a leak trace kit, put the dye in the oil run it and use the included black lite to trace the source, stops guess work.

[J. Gomez]

Garrett,

Is the oil filler cap clean and unrestricted?  A dirty or restricted cap will block the air flow to enter the crankcase forcing excessive pressure at the vent pipe, as Roger sated excessive blow-by.

Just as a precautionary step it would be a good idea to also clean the vent pipe.

Good luck..!

[Cadillac Nut]

Thank you everyone for your input!!  I have e-mailed him your ideas I will let you know what he says.
Glen--I think outside the blowby tube, but I will check with him.
Roger-- I think he has checked this, but I'll ask him.
Grant--  That's a good idea! 
Dave--  He may have done this already, if not, it should be the first thing he does. 
J. Gomez:  The engine is about as clean and detailed as the come, I would think the filler cap and vent pipe are clean.  But I asked him about it, to make sure.

Thanks again,

Garrett Baker

[Cadillac Nut]

Hi Guys,
The oil leaks outside the blow by tube.  Oil filler tube and filler cap are very clean and the blow by works as it should.  He's thinking it's the rear oil gallery plug like Grant mentioned, the car has leaked since new.  Dealer replaced the pan gasket and main seal in 1958 and that did nothing.  Thanks again to everyone for their help

Garrett