Have a grease-like substance on my front brake disk

I have a small amount of something that looks like grease on my front brake disk? My bike is a '73 Mk III. It has never happened to me before but lately (in cooler weather - 50s and 60s) a bit of something black like grease was deposited on the outside surface of the front brake disk. It obviously happened while moving since it was slung pretty evenly across the face of the disk. There wasn't much on the disk but enough to affect the braking. I have sealed bearings in the front axle and it doesn't appear to be brake fluid as I'm not losing any and have plenty of lever. The calipers were rebuilt a few years back and I am running an aftermarket master cylinder. Any thoughts would be appreciated. Thanks, Jim

Fork seals weeping on the fly have done this greasy griming to mine. Look at engine front to see if similar layer.

hobot said:

Fork seals weeping on the fly have done this greasy griming to mine. Look at engine front to see if similar layer.

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Or even the little fork drain screw seeping a bit.

Yellow_Cad said:

I have sealed bearings in the front axle.... Thanks, Jim

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Maybe they "used" to be sealed.

Brake fluid mixed with brake dust?

Have you checked the master cylinder level? Clean the disk. Put a clamp or wrap an elastic cord around your brake handle to hold pressure then come back later and look for weeping. That will check one system anyway.

Russ