Rear Brake Contamination From Axle Oil

It is a common problem with Magnettes and other cars that use the same rear axle. The oil has a tendency to find its way into the brake drums and gets spread liberally over the linings, leading to potentially dangerous brake fade and imbalance.

Sometimes it is a simple case of oil seal failure, necessitating a replacement. However, there are two maintenance items that are easily forgotten and can promote this problem if neglected.

Axle Casing Vent

At the top of the axle casing, there is a small breather cap designed to let air out without letting dirt in. This needs to be kept clean and clear. Its purpose is to prevent air pressure build-up in the axle casing. If pressure is allowed to build up, it will force oil out of the axle ends past the oil seals and into the brakes. The breather can be unscrewed for cleaning.

Axle Drain Holes

I was reminded of this item by Paddy Reardon's recent forum posting. He had suffered brake contamination and in the process of the strip-down and clean-up, he had discovered one of the designer's little known solutions to the problem: 

"While I was poking around with the hub, I noted the casting has an oil throwing ring on the inner edge. This fits into a catching plate on the brake backplate and drains through a drilling at the 6 O'Clock position - Both of mine were blocked with road dirt. If these were clear, I would have saved on a set of new brake shoes and a lot of mess. 

There is nothing in the manual about the drain. It seems to me  to be very important it is kept clear. Otherwise there is nothing preventing the brakes from getting a good lubrication, should the seal fail. 

The hole can be cleared externally with a 3/32" (2.5mm) drill bit."

As Paddy points out, these holes are designed to drain away any small quantities of oil that get past the ring seals but are very prone to blockage from road dirt, especially if they are sticky from a bit of E90 weeping out. As part of a good maintenance routine, you need to get under the car and give the ends of the axle casing a good clean with a wire brush. At the bottom of the flange holding the back-plate, you should be able to see the drain hole. A bit of wire or, as Paddy suggests, a drill bit will be enough to poke the hole through and ensure it's clear. Check your U-bolts for tightness while you're down there!

If you ever need to extract the half-shafts (e.g. to remove the diff or "pumpkin"), you will find a paper oil-seal between the flanges on the shaft and the hub. This is best replaced rather than re-used and I usually put on a smear of non-hardening sealant. Later cars (from axle number 18835) also have a rubber O-ring that sits in a groove on the hub. Getting this to stay put while you reassemble the flanges can be challenging. The air was blue last time I did the job because the O-ring was slightly small and kept popping out just as I offered up the shaft. I had to stretch it a bit before it would sit still long enough for me to join the flanges. This modification can be fitted to older axles simply by changing the hub for the later type.


#1 Trevor Jones 2015-01-27 18:23
Good article Malcolm, well done
#2 Guest 2016-03-13 10:13
Your article brought these holes to my attention during my you say, they aren't noted in the manual, (although it's now been revised by myself!)

Also check the main breather on top of the case...mine too was blocked. :-)

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