Aftermarket Rear Master Cylinder Replacement Guide $10-$25

[The Mighty Gryphon]

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. 

[Laitch]

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.

[szabgab]

   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!

[sammysnail]

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.

[szabgab]

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...

[JerseyBrick88]

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!!

[Martin]

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 1482 times)
Reservoir Hose 2.png (38.08 kB . 768x576 - viewed 1435 times)

[johnny]

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

[The Mighty Gryphon]

Jo, I am with you on supporting the c***sucking Chicoms, but sometimes you just gotta look at how much 5.56 and kipper snacks you can buy with the $250 you save by purchasing a $13 master cylinder.

The Chicoms only get to add 2-3 bucks to their bioweapon projects on one of their master cylinders vs. at least $100 to the Wehrmacht when I buy the Motorrad part.

Update on my retrofits:  Right now I have done 5 of them, and they are all working well with absolutely no issues.  The first one I did, on my K75RT, now has a bit over 30,000 miles and is still able to stop the bike at parking lot speeds without the front brakes.  There are no issues whatsoever with bleeding/flushing, and except for this thread coming up every so often, I never give the rear brake a thought.

[beemuker]

a brembo, pricier than the China, or is it made in China?
https://motowheels.com/i-23900691-brembo-rear-brake-master-cylinder-11mm-piston-top-entry-gold-color.html?gclid=CjwKCAiA-f78BRBbEiwATKRRBHp-Q5RjkQu7oEVWnImJqT6_8Hw9M06NyUEcnkhNu-ucBEFaLafCdxoCLCQQAvD_BwE

[The Mighty Gryphon]

That's not the "official" Chicovid master cylinder.  No shaft, the mount holes aren't slotted, and the body is different, not to mention that there is no banjo for the line. 

[Steviescumbag]

Hey guys.

I have a 1992 K75 Rt (non abs) and I recently switched my rear master cylinder. My current problem is that the brake like that connects from the master to the calliper has different threads than the banjo bolt that came with the master. Where and what are the names (sizes) of the 2 threads and where can I find an adapter/extension . Or should I just replace the whole line with a banjo brake line?

How did you guys solve the problem.

[Martin]

I had the local Brake specialist shop make me up a new line. The going price is around $50.00 - $60.00 Au.
Regards Martin.

[Steviescumbag]

Hey,

I got a 92 k75 RT (non abs). If someone could tell me what is the size (name) of the banjo bolt that would fit into the original bremo rear calliper.

[rbm]

The original fitting on the OEM master cylinder is a straight through connector, it's not a banjo bolt style connector.



The threading on the OEM rear master cylinder is M10 x 1.0. 
Apparently, the banjo bolt that is supplied with the Chinese rear master cylinder is M9.75 x 1.25
Also, @sammysnail mentioned that he reamed out the threads on his Chinese rear master cylinder to M10 x 1.25 using a tap.  Careful to check the threads on the MC supplied to you first (if you bought the Chinese MC) before attempting this modification because you have to keep the same thread pitch.

[rbm]

Hey guys.

I have a 1992 K75 Rt (non abs) and I recently switched my rear master cylinder. My current problem is that the brake like that connects from the master to the calliper has different threads than the banjo bolt that came with the master. Where and what are the names (sizes) of the 2 threads and where can I find an adapter/extension . Or should I just replace the whole line with a banjo brake line?

How did you guys solve the problem.

@Steviescumbag, I had my lines custom made by an Ebay seller way back when.  The lines were very well made and nicely priced.    The seller's name is Raymond C and his Ebay name is coastguard1975.  He is Toronto-based if I recall correctly and I was very pleased with his service.   He uses Apex fittings and hoses and he can manufacture lines to order if you give him the specifications.  I just checked Ebay and he is still advertising.

This is a picture of the original OEM rear brake line and the line manufactured by coastguard1975:

[sammysnail]

Long time no post.

UPDATE:
My Chinese master cylinder has given no trouble whatsoever.

.

[Laitch]

UPDATE:
My Chinese master cylinder has given no trouble whatsoever.

Thanks, sammy.

[The Mighty Gryphon]

Thanks for reminding me, Sammy!  All three of the ChiCom master cylinders currently in my garage are still working perfectly.

[The Mighty Gryphon]

Another year, another 5-6,000 miles.  The ChiCom master cylinders are still doing a good job of stopping my bikes.   The RT now has 20,000 miles on it's M/C, and the RS has 14,000 miles. 

IMHO $30 well spent.

[frankenduck]

What are the threads on the pushrod? M6x1.0? M5x0.8?

[The Mighty Gryphon]

M6 on the 4 units I used.  I just used the nuts they came with. 

I didn't need to worry about the threads because I cut off the shaft and put an indent in the end and set one of the nuts to make a pocket so it could interface with the fitting on the pedal.  Works flawlessly.

[frankenduck]

I thought it might be easier to install a short threaded standoff on the end of the pushrod in order to keep the brake lever pin aligned.

[The Mighty Gryphon]

Not really necessary.  Just makes the job a tiny bit more expensive.

One thread exposed on a nut that is locked with the other nut is plenty to have a reliable positive engagement that will work for years and thousands of miles.  The counter bore I put in the end of the shaft where I cut it off is more or less an OCD farkle.  I'm pretty sure that most of the people who have successfully used these master cylinders didn't counter bore the shaft.

[Laitch]

I reckon Revzilla's annual OCD Farkle Sale will be held soon. Discount Code:morecowbell