Looking for rear non abs 13 mm master cylinder for my 1987 BMWK 100rt

[Brian Kohn]

I have rebuilt the rear master cylinder on my 1987 k100 several times with same result. Rear brake locks up within 75 miles of operation . Two different bmw shops have rebuilt it with same result. They suggest replacing the master. I have checked everywhere boneyards other dealerships BMW North America all say not available any ideas on if it is the master or where I can find a replacement. The bike is in beautiful contion & runs great. Just not stopping to goop

 

[98lee]

Brian,

Three things can cause the symptoms you are describing; a breaking down of the inside of the hose between the master cylinder and the caliper, or too much preload on the brake pedal not allowing the piston to retract past the bleed back hole, or a missing pedal return spring.

 

[51797]

First go through what 98lee suggests. Make sure the caliper pistons aren't binding.
If all else fails try euromotoelectrics.com out of Denver, Co. for a master cylinder.
I have used them for quite a few items on my old K-bike with good results.

Frank

 

[Brian Kohn]

Brian,

Three things can cause the symptoms you are describing; a breaking down of the inside of the hose between the master cylinder and the caliper, or too much preload on the brake pedal not allowing the piston to retract past the bleed back hole, or a missing pedal return spring.

Thank you. We thought the tiny bypass hole has been plugging up. There is no way to access that hole . Iwill replace the hoses & see if that works.

 

[Brian Kohn]

First go through what 98lee suggests. Make sure the caliper pistons aren't binding.
If all else fails try euromotoelectrics.com out of Denver, Co. for a master cylinder.
I have used them for quite a few items on my old K-bike with good results.

Frank

Thank you. BMWNorth America put me in contact with euromoto but they no longer have them.. I am going to replace the hoses as Lee suggested

 

[98lee]

Brian,

Make sure there is a tiny amount of freeplay between the pedal and the pushrod that goes into the cylinder and that the pedal return spring actually returns the pedal to the upper stop.

 

[oldcamper]

Thank you. We thought the tiny bypass hole has been plugging up. There is no way to access that hole . Iwill replace the hoses & see if that works.

I’m not familiar with your specific master cylinder but on other BMW/Brembo rear brakes the tiny bypass hole is accessible by removing the fitting that goes up to the reservoir. You will likely need to replace the rubber fitting that holds them together as it doesn’t separate easily but that is a standard Brembo part.

Bevel Heaven Vintage & Classic Ducati Restoration & Spare Parts (925)798-2385

These elbow kits will fit all remote reservior BREMBO master cylinders. They are not too difficult to install but if you have an obsolete master cylinder and your elbow is broken and leaking, here you go, you finally have an option. After removing the old elbow and oring seal thing, lube up...

store.bevelheaven.com

 

[redlegrider]

If the pros can't fix it and you've tried everywhere for a replacement, might be time to replace it.

 

[98lee]

If the pros can't fix it and you've tried everywhere for a replacement, might be time to replace it.

They are NLA.

 

[oldcamper]

I’m pretty sure it can be fixed by simply removing the fitting to the reservoir and opening up the (likely) plugged bleed hole in the master cylinder. There are TWO holes under that fitting. The smaller one towards the brake line is usually much harder to see. It’s some work getting the fitting apart as the rubber grommet gets hard after 30 years, but a replacement Brembo fitting is available (see previous post).

There are also shops that rebuild NLA BMW master cylinders, albeit at greater cost. One example:

Motorcycle Brake and Clutch Cylinders Sleeved and Rebuilt

Motorcycle Brake and Clutch Cylinders sleeved to original size and / or sleeved and rebuilt. Call 631-369-9515 or Email info@AppleHydraulics.com to order

www.applehydraulicsonline.com

 

[oldcamper]

If the master cylinder is Magura, it may be harder to get parts for it. In that case I’d contact Apple Hydraulics (see above) for guidance.

 

[Brian Kohn]

If the master cylinder is Magura, it may be harder to get parts for it. In that case I’d contact Apple Hydraulics (see above) for guidance.

That fitting is almost impossible to remove. The Brembo replacement fitting was the wrong size I will check with Apple & contact Brembo direct. Redlegrinder option is terrible. Not an option for such a fine piece of machinery.

 

[51797]

Just going through euromotoelectrics.com site looking at other items and came across a 16mm rear master cylinder that appears from the picture to be a match for the 13mm. In stock and fits a later K1100. I don't know how this would affect braking. But if you can't find a 13mm you might call them to see if they can answer that question. P/N 34 31 1 458 142.

Frank

 

[98lee]

It would result in the pedal needing to travel less, but requiring more pressure from your foot. Might not be a bad solution depending on friction coefficient of rear pads among other things.

 

[Brian Kohn]

Thank you Frank & Lee for all the info . I will replace the hoses. I spoke with Apple hydraulic they wiinot rebuild the Magura master because the inlet plug is impossible to remove without breaking . He did say some of the rebuild kits he was found have the wrong size piston causing this problem. I will try to get a rebuild kit from him. Very helpful, knowledgeable, & willing to share. Thanks again for all the feedback!