The first thing, of course, is to buy a new seal. A good source is Olcar Bearings and Seals, Southern Pines NC, 910-693-3324 (no web site or email). USA Parts Supply and others sell it too. There are 2 types of pinion seals on 1941-56 Cadillacs, which are unrelated to year or model; some are one type, some are the other. If I remember, the difference is that one has a narrower outside diameter than the other because it is in a “cup” of some kind. Call Olcar for details. To save time, I bought one of each type and returned the one I didn’t use because I couldn’t figure out which one I had before removing the old one.
To take the old one out you have to get the rear of the car well up in the air. It can be done on the ground with jack stands, but it is best over a pit or on a lift. You’ll need a large socket wrench, breaker bar and torque wrench, probably 3/4″ drive, to turn the pinion seal nut. You also need a large pipe wrench or large monkey wrench or some other tool to hold the driveshaft yoke from turning while you turn the nut. With another guy tightly gripping the wrench on the yoke–or you can brace the end of the pipe/monkey wrench against the floor or frame of the car–you remove the nut and then the yoke, and either pry out the seal or use a seal remover to get it out. You drive in the new seal with a large socket and tap the yoke back on.
Now comes the hard and tricky part. You must “pre-load” the bearing behind the seal. To do this you torque the nut to a very high torque rating as per the shop manual. You then measure the load on the bearing by rotating the pinion shaft using the socket and breaker bar, and measure the pounds of “pull” you need on a small spring scale as per the shop manual. You may have to undo the nut and re-torque it to get it right.
Since you have to get the back of the car reasonably high off the ground, need heavy tools, must get the pre-load right and probably a 2nd guy to help, it may be best to have this job done by a professional. I’ve done it three times over the years on 3 different ’49 Cadillacs, including my own. However, each time I was helping friends more skilled than myself who had the right tools and knowledge. The first time was on the ground, the second time was using a pit and the 3rd time was on a lift. The 2nd and 3rd times were, of course, a lot easier.
