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83 K75S - Rear Brake Master Cylinder

Is it normal to have some hydraulic fluid in the boot area

or is that evidence of a small leak from an impending failure of the o-ring on the piston of the master cylinder?

No. The boot is to keep dirt out, not fluid in.

Yes. Replace the master cylinder, unless you know how to rebuild it.




:dance:dance:dance
 
Yep, replace/rebuild the M/C.

If you ignore it, you risk loosing residual pressure which means you have to pump the pedal to get firmness. At worst, she just goes and you loose the rear brake.
 
I have a similar situation on mine. I took it apart ,cleaned it, rubber looks good. We'll see. Haven't had a chance to test it long term, so far it is ok. It is worth a try
 
My rear master cylinder blew up a couple of years ago while I was riding. The brake fluid spray got all over the rear shock piston and destroyed the shock from the inside. Had to replace both the MC and the shock.
 
Rear Master Cylinder

Rear master cylinder on my 90 K75 has been leaking a small amount for the last couple of years. I add a little Dot 4 couple times a year. Bought the rebuild kit last year but have been putting off doing it. The K75 has been moved to 2 Nd. place after buying a used 08 F800ST 2 years ago. K75 has about 153,000 miles on it but only got about 2,000 miles last year. Trying to decide if it's worth doing the rebuild or if when i tear it down I
will find the bore all pitted and i will have wasted my time & the $75 price of the Kit. Has any one had any
luck rebuilding one??

Bill
 
Rear master cylinder on my 90 K75 has been leaking a small amount for the last couple of years. I add a little Dot 4 couple times a year. Bought the rebuild kit last year but have been putting off doing it. The K75 has been moved to 2 Nd. place after buying a used 08 F800ST 2 years ago. K75 has about 153,000 miles on it but only got about 2,000 miles last year. Trying to decide if it's worth doing the rebuild or if when i tear it down I
will find the bore all pitted and i will have wasted my time & the $75 price of the Kit. Has any one had any
luck rebuilding one??

Bill

Given how easy it is to open the cylinder it might have been a good idea to look at it before buying the kit.
 
Given how easy it is to open the cylinder it might have been a good idea to look at it before buying the kit.

Hind sight is always 20 20 !!But I usually want to have parts in hand before starting a job. I got motivated enough last Friday that I pulled it apart & Installed the kit. Cylinder had a few shallow pits in it. Will have to see how long it last. Did about a 40 mile test ride. So far no leaks yet. Job only took about 3 Beers to complete.

Checked on Beemerboneyard web site & they show a new Master cylinder for about $130 some dollars. Showed used ones out of stock.
Bill
 
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Care to share the "logic" behind the "sitting for a few months will cause brake systems to leak" theory?:scratch



:dance:dance:dance
 
So what is the acceptable amount of leakage on YOUR BIKE?

How does that compare with BMW's recommendation?

What would be the result of a failure on that particular system?

A leak is a warning. Will you heed it?:dunno

New brake seals in undamaged bores do not leak, even in cold weather. Worn or damaged seals or bores do.



:dance:dance:dance
 
That's actaully a very good, dead on answer.

Most hydraulic systems will leak in the absence of residual pressure. It's usually due to lack of pressure to keep the cup expanded. Shrinkage of the seal, not so much but rather, any seal, requires a fluid film to be effective.

Brake systems though, I tend to be over cautious and that is just my advice but if if you are comfortable and the machine stops when you need, perhaps I'm being a bit of a Granny.
 
Ok, my answer, just from my experience and not a fact to be held as the truth in cour

So what is the acceptable amount of leakage on YOUR BIKE?

How does that compare with BMW's recommendation?

What would be the result of a failure on that particular system?

A leak is a warning. Will you heed it?:dunno

New brake seals in undamaged bores do not leak, even in cold weather. Worn or damaged seals or bores do.



:dance:dance:dance
No amount of leakage in a brake system is acceptable to me. I never had the luxury to see if that little "wetness' would stop. I was working on valued customers machines. I couldn't gamble.

BMW don't know SFS.

The results is the same as unforeseen rupture of a brake line, a skid mark in your underwear at the best.

A leak is a warning. It depends on how much patience you have and an understanding.

ALL NEW SEALS LEAK. There is another new topic and thread right there.
 
I am also familiar with powered hydraulic systems. (Military aircraft and industrial) The one thing that all your examples have in common and are not present in a brake system is a running PUMP.

Minor leakages (drops) will not effect the effectiveness of those systems as any oil leaks out that was supposed to lower the ramp or landing gear is replaced by the pump from the reservoir. The ramp or the landing gear will continue to lower until they reach their limit switches.

In a brake system, the "pump" is the master cylinder. Instead of running and providing what ever fluid is needed, it is limited by its single stroke displacement. If ANY brake fluid bypasses the seals, the resultant output is reduced. This causes the pads to have less travel/pressure available.

With a bike brake system, we are talking about less than 1/2 thimble full of fluid displacement. A few drops bypassing the seals is a huge percentage compared to the large systems you referred to. If your master runs out of travel before you stop, you may not like the outcome.



:dance:dance:dance
 
Dudes, slow down a little.
This aint jo mamma.
If the leak is acceptable to you there is nothing that anybody can say to make you think differently, but they might disagree, that is just the nature of the conversation.
My 2 cents, rear brake small leak Ok with me, front brake must be 100%
 
"Lee, re-read my post above again, this time slowly. ;)


Now granted I am the new guy here and I don't know the history but this post definitely has attitude behind it.
The easiest way to ignore somebody is to just ignore them, it requires no energy.
 
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