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Master Cylinder Reservoir Won’t Seal

Clay Bastian
New Member Expired Membership

I’m restoring a 1974 R75/6. Is there a way to seal the bottom of the reservoir to the cylinder?  I tighten the single bolt as much as I dare, but fluid leaks out as soon as it’s poured in. It worked fine the last time it was on the bike. 

Topic starter Posted : 12/10/2020 11:21
Richard Whatley
Member Moderator

1. There is a, sometimes hidden, o-ring between the reservoir body and the master cylinder body. You'll need to locate that part and replace it.

2. In order for the o-ring to make an effective seal, all 3 parts need to be surgically clean. 

3. There are both metric and inch-based o-rings. Sometimes I have better luck installing the slightly oversized (in cross section) version from the opposite measurement system. An o-ring assortment box is a great thing to keep around.

Hope this helps.

[color=blue]Don't hide 'em, Ride 'em !!

Posted : 12/11/2020 07:07
Doyle Smith
New Member Expired Membership

When I rebuilt my R90S master cylinder the kit did not include a new o-ring for the reservoir. I took the cylinder and the reservoir to a knowledgeable old time auto parts store and the counterman found me a blue colored o-ring that worked perfectly. It was of some sort of special rubber perhaps Viton designed for a/c systems. Try that and if you fail email me offlist and I will try to help you.

Posted : 01/15/2021 21:51
tom Lyons
New Member Expired Membership

suggestion: when you find a O-ring of the correct fitment dimensions, you'll want to test its compatibility to brake fluid. let it soak overnight in brake fluid then look carefully to ensure it isn't decomposing.  Some o-ring materials are impervious to DOT4, some are not. Always better to find out before refilling the Reservoir and putting the gas tank back on.

Posted : 02/08/2021 21:17


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