The Stovebolt Site Forums The Shop Area Driveline Leaky Drive Line---Oil everywhere

I'm having several leaks on my driveline 1940 3/4 ton special, this model was equipped with the big 14inch drums. Anyway I was noticing an oil like substance dripping down my rear tire. I thought it was the brake cylinder leaking but someone suggested it might be the seal on the real axle.

What do you guys think brake cylinder or axle?

The two smell entirely different. If you have brake fluid on the rim, it will probably take the paint right off. Also, you don't have a lot of spare brake fluid in the master so a leak will take the level down quickly.

What is the part/gasket called that would be the issue in the axle? Now that you mention it I don't think its brake fluid either

Floating axle or plain? Floating axle the oil seal is in the wheel hub. Plain axle the seal is in the housing just behind the brakes. Felt or leather in a metal cover.
Floating looks like this
http://chevy.oldcarmanualproject.com/shop/1938/38csm096.html

It looks like a plain axle, there is no protruding hub like in the picture. Does anyone know if they reproduce these anywhere, would NAPA have them?

OK my shop manual says this...its starts of by saying the 1940 car models are semi floating axles with hypoid ....then I go to the truck section and its says 1/2, 3/4 and 3/4 special are the same axle as the car just heavier construction. What do you think they mean by they, I guess I don't know what a semi floating axle is or the correct oil seal looks like

Assuming your leak is from the axle seal, be sure your axle vent is clean and not plugged. As you drive, heating up the rear end, your vent has to breathe, otherwise pressure will build up and push gear oil past the seal. I had that happen to my '51 half ton. The vent was plugged with old hard dirt, grease and paint. After cleaning up the mess on the brakes, I had no further problems and it did not require replacing the seal. I got lucky, maybe you will, too.

The way I found this out was I noticed the leak after going for a drive. I was curious if I had lost a lot of gear oil, so I opened the fill plug on the differential cover while it was warm and when I did, I got a rush of air. There was actually quite a bit of air pressure indicating that the seals were still working. Of course, they are not designed to hold air, so gear oil was pushed past one of the seals.
Kent

You are correct, 1940 3/4 ton specials are semi-floating and almost identical to the 1/2 ton axles. The same axle seal and similar bearing is used from at least 1939 through 42 1/2 and 3/4 ton and 1/2 ton through 1962. You will find the axle seal available through most of the venders that advertise and support this site. I don't see them listed at NAPA. Not cheap, usually around $25 each. You may also check to see if the toque tube seal is leaking, letting excessive transmission fluid leak into the differential housing and overfilling it.

OK don't laugh, but I had similar symptoms and it turned out to be the hillbilly neighbor's dog using my tires for a fire hydrant. Yes it was oily to the touch, and had an unpleasant smell, but quite different from gear oil. I did NOT confirm this with a taste test, in spite of the insistence to the contrary by my in-laws!
Carl

Where would I find that vent your talking about?

The vent will be on top of the axle between the differential and backing plate. On my Chevy trucks it is on the left (driver's) side. It will either be pressed in or threaded ( 1/8 pipe thread). My '42 did not have anything but a hole when I bought it.
Kent

I have two 1940 3/4 ton differentials. On one, the vent is located, like Kent suggested, on the top of the housing on the drivers side. On the other, it is located on the top of the torque tube. Both are press fit.