Master cylinder cover "sweating"?!?

A few times during a relatively high speed highway run on the 73 Commando today (sustained 4500 RPMs) I stopped and noticed "moisture" beading up on the alloy master cylinder cover. Appeared to be brake fluid. WTH? Is the cover vented in some way? Porous? Note, I barely used the brakes during these runs - just to park during stops. Weird.

BrianK said:

Is the cover vented in some way?

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The airspace below the cap must be able to vent to atmosphere to allow for changes in the reservior fluid level (the black plastic caps certainly have a vent hole intended for that purpose) but the diaphragm should make an airtight seal to the master cylinder to stop any brake fluid escaping and also prevent moisture from getting into the system?
Are you sure you haven't overfilled the reservoir? Is the diaphragm still folded (presuming it's the later type which you fold up before fitting)?

Thanks Les.

Am I sure I haven't overfilled?!?! Me, Mr. "tighten til it strips, then back off 1/8 turn?" Hey, if a little is good, then...?

That's probably it right there. I'll check it out in the morning.

Thanks as always, Les. - Brian

My 2 Combats with the Al cap have a pin hole in center of cap.
That's about only place I've never had a brake fluid leak.
Maybe maybe your rubber air guard has a leak.
Be alert to watch for mist or small drops of fluid on paint
spread by travel wind.

hobot

hobot said:

My 2 Combats with the Al cap have a pin hole in center of cap.
That's about only place I've never had a brake fluid leak.
Maybe maybe your rubber air guard has a leak.
Be alert to watch for mist or small drops of fluid on paint
spread by travel wind.

hobot

Click to expand...

Thanks Hobot. Yeah, my cap - this is an 850 - has a little mark in the center that I've never thought was a real "hole" but maybe it is. I gotta check all this out in the morning. Never an end to the learning curve with a Commando!

Yes, the joys of discovering hidden Norton details.
These can get crud crusted over to block much light
showing through. I got a double take head cock after a polish
inspection.

Putting fluid in or just checking it gives me creeps handling
the rubber cover and pouring it.

hobot

My MKII also weeps through that little hole in the cap. Dunno why, as I'm very careful about tightening it and the rubber bellows has no tears. Drives me nuts.

maylar said:

My MKII also weeps through that little hole in the cap. Dunno why, as I'm very careful about tightening it and the rubber bellows has no tears. Drives me nuts.

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The original "bellows" diaphragm has a chamfered lip, which probably doesn't make a particularly good seal between the squared-off faces of the master cylinder and cap (and washer) and I think it could even be squeezed out of the joint by over-enthusiastic tightening of the cap?



The later design appears to be better, as the lip has parallel sealing faces.


When the later type diaphragm is installed, it should of course be folded to look like this:


And not installed like this: