Brake Master Cylinder: Seal Orientation. ETZ 250

[nice2day]

Hi Chaps.

just a simple question as regards the seal in the master cylinder

Thanks in anticipation:

Les

[Guesi]

You will get a lot of answers in case you tell us about what model you are talking !

Or is this a kind of: guess what bike I have

[nice2day]

It's a 1988 MZ ETZ 250...The master cylinder has the round white plastic reservoir with black plastic cap....
Part number 30-24.080...Item 5

Thinking about the way it works a little more I reckon its likely the smaller diameter part faces the pressure... IE: the brake cylinder.
100% confirmation would put my mind at rest though.
Thank you Guesi.
Les

[Blurredman]

I have recently done this.

The side of the seal with metal spring showing (in the concave side), faces the cylinder. Oil seals normally have the spring on the 'wet' side of the required sealing area.

[Guesi]

This is how it should fit....

[nice2day]

Very many thanks Blurredman and Guesi.

In reply to Blurredman. I may have confused you with my description "OIL Seal"...perhaps I should have used the MZ description "O ring" but I didn't as it's NOT an O Ring!...A better description would be "Piston Pressure-Seal" or similar. There is an oil seal at the lever end however. (Item 3 in the parts diagram above)

In reply to Guesi...Yes that's the part and many thanks for actually showing the picture of it in situ on the piston. It seems obvious now looking at your picture and indeed after checking the new item again much more closely. It is clear there is a taper on the circumference which gets pushed outwards by the pressure to further seal against any oil being passed by it. I did not notice this originally.
Fitting it will need a lot of care to avoid folding back the edge so I will use a slight rotating action as I push it into the cylinder.
Hopefully this has also helped other restorers too....This is why I have amended the Title just now.
I am very much obliged.

Les

[Blurredman]

Oh that seal, yes, sorry

I must admit I just installed new one in same orientation that the old one came off. But I did use Gussi's image as reference to double check.

[nice2day]

Well I fitted the piston seal but now I'm not sure about the outer lever end seal

If you look at the photo, the seal appears to have the knife edge part upper most so it should fit towards the oil (inwards) BUT the taper on the outer rubber diameter looks as if it won't press in unless you fit it the other way round as the taper seems to favour pressing in with the flat plain side first...Sorry to ask again but can anyone give me an assured answer please?

Thanks in advance....

lever end seal.JPG (13.7 KiB) Viewed 9117 times

[Guesi]

The top side of the rubber comes in direction to the inner side of the brake.
The flat part (bottom in the picture)comes outside towards the lever.

But this rubber won´t last very long.

I prefer double lipped oil seals (which have to be brake fluid resistant)...

Like this set:

https://www.guesi-motorradteile.de/inde ... 1/c/_/_/?_

[nice2day]

Once again Guesi, you have been a tremendous help to me and I am very grateful. I wish I had known about your double lipped seals before as I have already bought all the old style parts, but then again I have another master cylinder to repair so will remember to buy the kit from you when I do it. I also viewed your "Oil Seal fitting Guide" pdf, which is also informative.
Thank you very much Guesi.


Les