Front brake master cylinder leaking

[Dr Klawn]

I'll start of by saying that I've got a 1996 R1100RS. Got home from our state BMW club rally on Sunday and while washing the bugs off the bike noticed a dull spot in the paint below the right controls. Got to looking under the right hand lever and noticed some fluid and that most of the paint had bubbled up around the rubber dust boot on the master cylinder.

 

I got to taking it apart last evening which never seems to go as easy as "the book". Ends up that my brake line runs directly below the pivot bolt holding the brake lever so had to loosen the banjo bolt and rotate that (I assuming I'll need to replace the washers?) and then got the lever off. The dust boot was filled with congealed and regular brake fluid. The metal (aluminum) around the top of the cylinder walls was heavily caked with dry fluid and signs of pitting.

 

My questions are:

1. How do I get the piston out of the master cylinder? I've drained the reservoir so that I don't have fluid running all over the place. It seem like if I try and hook it or other method I'll only end up scratching the cylinder walls. None of the books diagram this area.

2. What pieces generally need to be replaced in these instances? It seems like this was a somewhat expected circumstance on the older oildheads.

 

I'd really like to be on the road this Saturday so any help would be much appreciated!

 

Thanks,

[Ken H.]

I've not heard of much success rebuilding them. I'd say replace the entire master cyl.

[Joe Frickin' Friday]

My questions are:

1. How do I get the piston out of the master cylinder? I've drained the reservoir so that I don't have fluid running all over the place. It seem like if I try and hook it or other method I'll only end up scratching the cylinder walls. None of the books diagram this area.

 

I have no experience here...but I'd think you could drive it out hydraulically. Plug the bleed hole at the bottom of the reservoir with something (a gloved finger?), and have someone else push a brake pad back into the caliper; caliper slave piston moves in, master piston moves out, and should move beyond the bleed port, if you've properly plugged the hole at the bottom of the reservoir. Be ready to catch the piston, and be ready to catch a whole lot of fluid (including possibly a jet of it when the piston comes free).

 

2. What pieces generally need to be replaced in these instances? It seems like this was a somewhat expected circumstance on the older oildheads.

 

Again, no experience here. Unless someone replies with experience, I'd say search the web for things like "master cylinder" "rebuild" "piston" and so on. See what general info you can come up with.

[ShovelStrokeEd]

BMW really screwed the older owners on this one.

 

The master cylinder typically leaks at the actuator piston due to corrosion. BMW never has accepted responsibility for this poor design and requires replacement of the fluid on an annual basis as the PM. In fact, the thing is just as likely to leak as not. I'm on my third on my '94 RS. It would be more but I layed the bike up for 3 years cause I was so pissed about this very situation. A new master cylinder is 196 bucks.

 

I think it is made by Magura and you can get a new seal and piston but I don't know the part numbers. Perhaps others can help.

[Dr Klawn]

Again, no experience here. Unless someone replies with experience, I'd say search the web for things like "master cylinder" "rebuild" "piston" and so on. See what general info you can come up with.

 

Thanks Mitch. I did a lot of searching over the last couple of days. Several responses were to pull the piston out and verify if there's any pitting, etc. Then clean up and reassemble. Made it sound like a simple job. When I push in the piston can see some of the cylinder wall surface and it looks OK. I just don't know if there are any other seals that should be replace inside the assembly. Maybe some of the mechanical / dealers out there may know this? The parts fiche shows a "rebuilt kit" part #32-72-2-332-037 but doesn't really indicate what all that is.

[smiller]

That part number is correct for the rebuild kit, which includes the plunger and rubber seals, new spring, cap, etc. As you have noticed it can be a PITA to get the old plunger out after things have corroded up but usually it can be accomplished. (Make sure that you have removed the retaining screw on the side of the master cylinder or I guarantee you won't get the plunger out.) If it's really, impossibly stuck I expect that Mitch's suggestion would work but if you do this... watch out and have a bunch of towels handy...

 

After disassembly you can clean up the inside of the bore with some fine sandpaper, like 400 or 600. Flush the bore out thoroughly with brake fluid when done so that it is absolutely clean of any abrasive particles, then reassemble. I found that I was able to reprime the master cylinder and get all the air out without a lengthy brake bleeding process. It's all a pretty simple operation if you go carefully and keep everything very clean (very important).

[mikefigielski]

Yep, that is the drill. Get the rebuild kit and make sure to take the retaining screw out. Then the piston should come out easily. Might want to soak the replacement pisto/seal assembly in new brake fluid overnight to make the seal more pliable. If the old piston is stubborn, a little compressed air will work, but be careful! BMW started anodizing the master cylinder bores on the later models, guess they realized the problem finally. I have rebuilt a few with good results (short term anyway).The rebuild kits are about $45 from Chicago BMW. Good luck!

Mike

[georgem]

Mine did not come out easy. I had to use a wooden punch and a SMALL hammer and TAP against the spring pushing the plunger deeper and letting the spring push it back out. This took quite a while, however it was non destructive. Also for half of the cost of a new mastercylinder, Mike Haven at MPH cycles will sleave yours with stainless. Permant repair. His link is attached. Real good people at MPH.

http://www.mphcycles.com/

 

Good luck.

P.S. DONT try to pull the piston out with a pair of needlenose pliers. MFU if you do.

[Dr Klawn]

Thanks for all the info!

 

Now that I understand further of how to disassemble this mess I should be able to break it all down this evening. I had not seen the retaining screw on the side. I did happen to find some good info on this out on the web with additional searching.

 

Check out http://www.largiader.com/tech/r11mc/

 

The Haynes book of lies and the Clymer Manual (does it lie too?) have absolutely no info on taking these things apart, aside from removing the lever.

 

I'll let you know what I find out. Some past threads that I found in the "search" zone had all sorts of cleaning, etc. if the components weren't pitted, etc. If not then I'll be getting parts Fed Ex so I can get out this weekend!

[Dr Klawn]

OK. So I got the rest of the M/C apart last evening. Not too hard when you understand the procedure The outer seal on the piston was fairly worn down and scored from the grime that had stuck to the leaking fluid and build up under the dust boot. After polishing the inside surfaces of the M/C with some 800 & 1000 grit sandpaper, I noticed some slight pitting from the corrosion. The depth isn't anything to snag a nail or anything on but I was wondering if the rebuild kit would work long term I polished the lightly corroded walls further to even out the pitting. Like I said, the pitting seems slight. Will the pressure of the M/C end up leaking again if I go this route? Or should I just replace the whole master cylinder....

 

Also, does anyone know if the replacement master cylinders have the anodized walls or are they the bare aluminum?

 

Thanks!

[georgem]

Yes it will probably leak again. I am not sure if the new on is anodized or not, it's just not worth the risk of trying to just "get by". After all, we are talking about the difference between stopping or not. A new master is about $300. You can call Max BMW and they will be able to tell you on the phone what the inside looks like.

[smiller]

Yes it will probably leak again.

 

Yes, in another ten years maybe. Rebuilding the master cylinder is not a 'get by' operation... done properly the result is an entirely reliable repair.

 

The reason for the problem we're discussing is that a film of brake fluid can be left at the very end of the piston travel in the exposed part of the cylinder (under the dust cap), which over time will eventually harden into a deposit, which then may in turn eventually damage the seal. If you simply pop off the dust cap and clean the area inside occasionally (maybe once a year when you do a fluid change) the crustiness will never have a chance to form and the thing will probably last forever.

 

Or you can throw $300 at it if you want, up to you...

[Stan Walker]

Learning how to rebuild brake systems by trial and error and email could be a "smashing" success story.....

 

I do agree that it isn't hard, but it is critical that you get it right the first time.

 

Putting that another way. If you have to ask on an internet list how to do it, you probably shouldn't be doing it at all. Some really bad information gets presented as absolute fact from time to time.

 

Stan

[Stan Walker]

I noticed some slight pitting from the corrosion

 

I would replace it if this were my bike. Sanding the pitting out increases the size of the bore. A bad idea on these tiny master cylinders.

 

Edited to add this note: This is the type of problem that the annual brake flush helps to prevent. Only you and your dealer know if they have been done faithfully.

 

Stan

 

[smiller]

If you have to ask on an internet list how to do it, you probably shouldn't be doing it at all. Some really bad information gets presented as absolute fact from time to time.

 

Now that I do agree with...

 

My comment assumed adequate mechanical skills on the part of the rebuilder. As always, and especially with brake systems, if you are unsure about anything best to have a professional take care of it. However, if done properly (a qualification mentioned in my post) the master cylinder can be restored quite successfully.

[Dr Klawn]

I appreciate all the comments. I've ordered a whole new master cylinder after thinking about it this morning. ($192.80 from Chicago BMW) I think the existing unit could be rebuilt but I don't want to risk it not lasting. I don't think the buffing out of the cylinder walls would enlarge the bore that much more than a few ten-thousands of an inch. Like I said the pitting is almost like a surface discoloration more than anything. The new unit is the "improved" design, whatever that means. I like the comment on pulling the dust cap yearly and wiping things down since we ride year round and get lots of water / road grime here in the winter months in the NW. BTW, the brakes have been serviced yearly since new.

 

As far as "lack of knowledge" or "asking by email" goes, well I'm not personally worried about my mechanical abilites. Just haven't taken a BMW unit apart yet (and hadn't expected too...). It's a fairly simple mechnical device and I have a clear understand that it is my wife's, and my, life on the line. I do however take a common sense approach to all advise given.