Author Topic: Aftermarket Rear Master Cylinder Replacement Guide $10-$25  (Read 35750 times)

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #125 on: October 07, 2019, 10:32:59 PM »
While pondering the hydraulic ratio between master and slave cylinders I drifted into looking at racing type thumb brakes (rear brake operated by the left thumb).

By my reckoning the new Chinese-made K100 rear master cylinder at 14mm dia. requires about 15% more effort than the original 13mm one to get the same effect at the caliper. Brembo thumb brake master cylinders are also 14mm.

 Unless riders have massively muscular thumbs, I guess that the caliper cylinders on race bikes must be of considerably greater diameter than those on a Brick.

I'm pretty sure that my master cylinder is 13mm replacing the 12mm original.  In about 20,000 or so miles of riding, the Chicom master cylinder has worked perfectly, being able to trigger the ABS on dry pavement, something the OEM master cylinder couldn't do.  Heck, the OEM was barely able to slow the bike down, even at parking lot speeds. 
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too
'89 K100RS SE

Offline Laitch

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #126 on: October 07, 2019, 10:57:44 PM »
Unless riders have massively muscular thumbs, I guess that the caliper cylinders on race bikes must be of considerably greater diameter than those on a Brick.
Thanks for this observation. I was unfamiliar with this type of braking system. Certainly, hoses and caliper pistons must be sized correctly to effectively accommodate an increased master cylinder bore, but it occurred to me that for the thumb to work efficiently with a braking system, compound leverage would be effective in lessening thumb effort found in a 1:1 system like the Brick's cable/handlever assembly. I found this photo of a multiple-jointed thumb lever system. It seems to confirm that notion.

  • Along the Ridley in Vermont.
  • 1995 K75 72,000 miles
I wept because I had no radials until I met a man who had no splines.
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Offline szabgab

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #127 on: October 17, 2019, 04:08:20 AM »

   My bike is a 1985 K100RS   with NO ABS, suffering from Magura seal failure.   

   The easy solution is, of course, to use the original hose and pipe, screw the hose into  an M10x1.0 to M10x1.25 adaptor,  use a thread tap to open out the thread in the hole in the master cylinder and screw the whole thing straight into the back of the new master cylinder

My rear master cylinder suddenly started to leak like a sieve. I have tried in vain to clean the inside of the bore (even though the rubber on the piston looks good enough), it still leaks. Anyway, I had this conversation bookmarked a while back (shame I did not buy a Chinese MC long time ago), so I have ordered one of these just now. My brake lines are still in good shape, but my bike is non-ABS, so I would need to replace the line to something custom made I guess, but Sammy's solution above gave me some hope of a cheaper solution. So my understanding is, the original line is m10x1.0 and the Chinese MC is M9.75x1.25? After re-tapping all it needs is this adapter to accommodate the old line? Thanks!
  • Budapest, Hungary
  • K75S 1985 model

Offline sammysnail

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #128 on: October 21, 2019, 02:03:38 AM »
That's what I found Szabgab. I ran an m10x1.25 tap through the hole in the new master cylinder (it is best to first confirm that the thread on the banjo bolt that comes with the new master cylinder is indeed 1.25mm pitch - my new master cylinder does not look like the ones pictured above and there may be variations between the different ones available).

Then I obtained from the local brake service an adapter, m10x1.0 on the female end and m10x1.25 on the male end. The adapter screwed into the master cylinder, and the hose screwed into the adapter. The adapter moves the hose back about 25mm, but I don't think that this will cause a problem. I just had a look at my old hose and it should fit well in the new position.

The bore of your master cylinder  is probably 13mm and the bore of the Chinese one probably is 14mm, so you might find that just a little more effort may be required at the pedal. (ABS brakes have a 12mm bore so they would require even more effort) That is the theory, but in practice I haven't really noticed any difference.

Regards, Sammy.
  • 1985 K100RS Second owner.
Bought an R75/5 new. Cruising speed 100 mph. Those were the days. Now the Brick will go faster, and I ride it slower (well, mostly).

Offline szabgab

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #129 on: November 03, 2019, 04:45:35 PM »
That's what I found Szabgab. I ran an m10x1.25 tap through the hole in the new master cylinder (it is best to first confirm that the thread on the banjo bolt that comes with the new master cylinder is indeed 1.25mm pitch - my new master cylinder does not look like the ones pictured above and there may be variations between the different ones available).

Then I obtained from the local brake service an adapter, m10x1.0 on the female end and m10x1.25 on the male end. The adapter screwed into the master cylinder, and the hose screwed into the adapter. The adapter moves the hose back about 25mm, but I don't think that this will cause a problem. I just had a look at my old hose and it should fit well in the new position.

The bore of your master cylinder  is probably 13mm and the bore of the Chinese one probably is 14mm, so you might find that just a little more effort may be required at the pedal. (ABS brakes have a 12mm bore so they would require even more effort) That is the theory, but in practice I haven't really noticed any difference.

Regards, Sammy.

Hi Sammy, thank you for your reply. I was going in circles in my head with this.... Later I thought why not give a ring to a brake line specialist, and guess what, a new line with a 90 degrees banjo on one end and an m10x1 thread on the other would cost me something like 8-9 dollars. That is a no-brainer, I will get a new line and no adapters, etc... Obviously this ship from China is indeed very slow and my original rear brake master is puking brake oil whenever I step on the pedal...
  • Budapest, Hungary
  • K75S 1985 model

Offline JerseyBrick88

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #130 on: May 06, 2020, 08:12:46 PM »
Long time lurker, first post. I want to thank Gryph, Martin, and the rest of you brave souls for testing out the Chinese mc hack. I followed in the footsteps and ended up working out great. Waiting for the new brake line with the banjo fitting to finish it but it came out great. Thanks again for everyone's input!!
  • Philly
  • 1988 k100rs

Online Martin

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #131 on: May 06, 2020, 08:45:14 PM »
WARNINGJB88 do not use the supplied hose from the reservoir to the master cylinder. It is not rated for use with brake fluid and can disintegrate.
Regards Martin.

* Reservoir Hose 1.png (35.73 kB . 768x576 - viewed 47 times)
* Reservoir Hose 2.png (38.08 kB . 768x576 - viewed 47 times)
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline johnny

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #132 on: May 06, 2020, 10:11:54 PM »
greetings...

the quicker you f-ers stop buying china junk the sooner the world be right again...

example... i coould have got a chinese made codpeice for my new job... butts i searched the conus over and ended up buying a usa made codpeice for 4 times the money...

when you buy chinese... commies own you... f-that...

j o
  • :johnny i parks my 96 eleven hundert rs motobrick in dodge county cheezconsin  :johnny

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