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Old 07-15-2011, 12:58 PM   #1
Xs395 OP
Joined: Jul 2011
Location: Chicago
Oddometer: 8
Eh? BMW R90S Master-Cylinder Rebuild Distress

Hey everybody, thanks for checkin in.

My father and I are currently restoring his 1976 BMW R90S with the Black with Silver Smoke paint. Our issue recently is that we are up to the point where before we can get the gas tank back on the bike and see if it'll start after our work, we need to get our master cylinder functional.

In the terms of our front brakes, we have replaced the brake pads, replaced the old hydraulic brake line hoses, and made sure the calipers are functional. What we came across is that when we try to bleed the brakes, there is a complete lack of pressure in the lines. We are following the procedure where you keep the bleed port closed until you have the brakes squeezed and then you bleed a little bit at a time, but instead of a shot of fluid, it just drips. We decided that our master cylinder was corroded enough to be in need of a rebuild anyway, so we got the fancy rebuild kit, runnin about $60-65.

It comes with a few o-rings, the internal piston, etc. The issue is that the actual rubber ring that maintains the pressure inside the master cylinder comes SEPARATE from the piston itself. This rubber piece is tight even getting on the very tip, let alone trying to work it over the next ridge towards it's home about a centimeter further up the piston.

Basically I want to know if there is a tool/technique to getting this ring on, because I don't want to make a silly $60 mistake due to impatience. I will try to include pictures.

Our next venture is to use dental floss to gently stretch it out to get it over the bump.

I'll post a picture of the master cylinder when I get home from work, but here are some bike pics.
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Old 07-18-2011, 08:39 AM   #2
Jim K in PA
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Welcome to the asylum. Nice bike.

Yep, those seals are tight, and I thought the same thing when I got the kit. I used a small chain saw carb screw driver to start them up over the piston. Use fresh brake fluid as lube. Make sure you get the orientation correct, or you will be doing it twice.

There should be two of those seal, no? Mine is a '79, so may be different.
Jim K in PA
1979 BMW R100T - Otto - Returned to the road - 5/12/11
1975 BMW R90S Hack - Olga - Olga's story - adopted 3/22/14
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Old 07-18-2011, 09:38 AM   #3
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Welcome to Advrider and the asylum!

It's been a while since I bought a rebuild kit, but it seems they always came with the seals installed. I sure wish they'd sell the seals separately - sixty bucks for a piston you don't need and a couple of itty bitty seals is a lot of money.

Dump the seal in some boiling water and it'll stretch easier. Put a thin coat of brake fluid or brake lube on it and work it on over. I've got a brass rod, machined to install orings on the Bing carbs - something like that would be perfect for this job. Slide the seal onto the rod, which is the same outside diameter as the largest dimension it needs to clear, the shaft is hollow to fit over the shaft - then butt the shaft with seal up to the seal stop and slide it over.

If you've got access to a small lathe it would be easy to make a tool like this.
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Old 07-18-2011, 11:28 AM   #4
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Dental floss

When I rebuilt mine, I took a long piece of dental floss and looped it through the rubber ring a number of times and then got the ring started on one side and used the dental floss to pull it over the other side. The floss was then easy to remove by cutting the loops and pulling them off the ring.
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Old 07-18-2011, 05:12 PM   #5
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You can make a tool that works very well for very little $. Get a small plastic funnel and cut the tapered ouitlet off so the big end is just slightly larger than the piston. You now have an installation cone. Put the piston in a soft jaw vise and push the big end of your new tool against the piston after starting the seal on the small end. While pressing against the piston the seal will slide up and on. No muss no fuss.

So much riding-so little time
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Old 07-28-2011, 05:59 PM   #6
Xs395 OP
Joined: Jul 2011
Location: Chicago
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Thumb Attempt 1

Finally getting around to slinging the ring over, using the boil to soften and then dental floss method. If that doesn't work we'll try other ways but we're confident it will work.
Tricky little piston.

Xs395 screwed with this post 07-28-2011 at 06:10 PM
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Old 07-28-2011, 06:40 PM   #7
Xs395 OP
Joined: Jul 2011
Location: Chicago
Oddometer: 8

Success, boiled it up, lubed it up, dental-flossed it up. There were two teals, as Jim K queried, I just didn't mention it because the second seal is no issue getting on the piston.

Next step: Getting the piston into the cylinder without stressing the rubber lips. We have an idea to use some sort or sleeved too to keep the flare tight to the piston until it is safely inside.

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Old 08-02-2011, 07:37 AM   #8
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When bleeding, go through a few iterations of the bleed cycle, then use a bungy to tie the brake lever closed, then leave until the next day and you should find that bleeding will be simple after that.

If you don't need the bike to be stock, using a 13mm twin pull handlebar M/C will give you very much better brakes, both in terms of absolute power and more importantly the old wooden feel of the original setup is gone.
R90s 1070 replica, R90/6, 1958 R50
1971 Commando Fastback
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Old 01-06-2012, 11:42 AM   #9
Joined: Aug 2011
Location: Bay Area - CA
Oddometer: 86
I have rebuilt mine twice now with no success.
I think the cylinder is pocked even though it looks okay.
This is my next step:
Fingers crossed.
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Old 01-06-2012, 05:29 PM   #10
Hawk Medicine
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But if youre about to spend $200+, you could easily buy the parts necessary to install a 14mm Handlebar MC and that absolutely would provide you with better braking.

Just sayin...

Hate rots the pocket that carries it.
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Old 01-06-2012, 05:40 PM   #11
Beastly Adventurer
Joined: Sep 2011
Location: Tulsa, Oklahoma
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everything you are going through is documented in my bringing back to life 74 R90S thread
this includes details on how source brake fluid resistant seals between tank and cylinder.
those seals are not available with purchasing an entire brake master cylinder for big $$$$.

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