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Master Cylinder Reservoir Won’t Seal

6 Posts
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Clay Bastian
Posts: 1
New Member
Topic starter

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. 

Posted : 12/10/2020 11:21
Richard Whatley
Posts: 2421

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
Posts: 10
Active Member

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
Posts: 5
Active Member

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
Jim Wilson
Posts: 206
Estimable Member

ok gents,


there is no hidden seal. in fact there are 2 seals under the whitish plastic reservoir. these are not so commonly sold with a rebuild kit, LOOK at the dealer selling you the kit and if they state [ 99%] when stated] say viton or buna . .. . you're IN Trouble.


brake fluid is particular in that you NEED - - -  MUST ONLY USE an EDPM material seal rubber.  ... If you don't at some point in time that seal WILL Fail.


pm me as i have a couple extra. 


While NBR and FKM have similar chemical resistance, EPDM is completely different. It is incompatible with petroleum (oil and gasoline), but it is compatible with alcohols (ethanol, methanol, and nitromethane), water, coolants (both ethylene and propylene glycol), and glycol-based brake fluid. Of all three rubber types, EPDM is the only one that should be considered for use anywhere in a braking system. (Of course there have to be exceptions -- the Citroen and Rolls-Royce high-pressure brake/suspension systems that use green hydraulic oil must avoid EPDM and should use FKM instead.)


This post was modified 2 years ago by Jim Wilson
Posted : 06/03/2021 15:10
Richard Whatley
Posts: 2421
Posted by: @1559

OK gents,

There is no hidden seal.

By "hidden" I only meant to infer there was a seal between the reservoir and metallic body that could not be seen unless you completely stripped the m/c down to bare bones. By "hidden" I meant to infer that you cannot short-cut the job by merely removing the piston assembly and only replacing the seals supplied with the rebuild kit. 


I will further admit that in my specific case, any o-ring that fits the groove provided works fine for me. But my personal case is special. I use DOT5 Silicone brake fluid which is not caustic to paint, o-rings or electrical equipment. 

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

Posted : 06/05/2021 07:39


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