Aftermarket Rear Master Cylinder Replacement Guide $10-$25

[frankenduck]

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.

Just for grins, I bought 5 stainless M6x10 standoffs at Ali Express for $1.72 to experiment with.
(https://www.aliexpress.com/item/3256803153745358.html)

Even if it's not necessary, I can also use one as a guide to center a drill bit in order to make the end of the pushrod concave.

[sammysnail]

Chinese master cylinder followup.
September 2022, and my rear master cylinder has become impossible to bleed. Looks like a seal problem, so I bought a new one via the usual source. About $20.
It arrived today, so I removed the existing one and examined it. Looked inside the bore and it looked OK. The cups looked OK. Reassembled it and played with it. The action felt a little grabby and the piston more often than not would not fully return to the rest position. This meant that the piston cup would not travel back past the small hole that feeds more fluid into the cylinder when the piston is at rest. This would cause my symptoms.
I suspect that when I adjusted the operating rod I did not allow any play, and that in fact I had adjusted it so that the piston was held off its seat with the feed hole maybe obstructed.  Vibration during subsequent use may have caused a tiny ridge to have formed  in the bore, and this ridge may have resulted in the piston sticking when not fully home.
Rather than fiddle around with the old master  cylinder I fitted the new one. Shortened the operating rod, tapped out the hole on the other end to m10x1.25, and left an obvious amount of play between the operating rod and the pedal. Problem solved. $20.

I thought that the last MC that I fitted was 11mm bore, so I bought that size again. Turns out that the old one was 14mm, so if the new one really is 11mm  I now have a higher hydraulic advantage. This means that for a given pressure on the pedal I will have greater pedal travel and higher fluid pressure. In the workshop the pedal travel seems the same. A road test will reveal all. Now, if it would only stop raining.

[The Mighty Gryphon]

Thanks for adding your update.  This thread will be more valuable as time goes on and more of the BMW master cylinders fail.

[SunnyPerth]

I to had to go this way, as the rear master cylinder from my project K100rs was cactus.

For those that going this way, here hopefully some more tips that help.
- I removed 63mm of the thread. Obviously if you have a different master cylinder this might be different for you.
- drilled out the thread a bit, to make it adapt the pin better, see image. To do this; I locked two nuts, and put them in the vice. This means there is no strain on the cylinder when drilling. A 7mm drill seems to be the right size.
- I also filed down the lock nut to give the rubber boot a bit more space && used thread locker, just in case..
- then I removed the little tab on the back of the cylinder, this means I could use the original brake hose (for now. ) (this bike has ABS)

Cheers!

[The Mighty Gryphon]

Nice!

[Gumbguy]

Thank you for posting this innovative hack! Ive scanned this thread and I don't see what specific master cylinder to buy. I see many in Amazon and eBay and I don't want to get the wrong one. Ive dicked around trying to make a Yammie one fit. I see this one. Is it a go? Thanks in advance!
🤔

https://www.amazon.com/Universal-Motorcycle-Cylinder-Hydraulic-Reservoir/dp/B07F2ZSQ54/ref=asc_df_B07F2ZSQ54?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80608063550932&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=m&hvlocint=&hvlocphy=&hvtargid=pla-4584207589829344&psc=1

[The Mighty Gryphon]

That looks like what I used.  If you look on eBay, you should be able to find the same thing in black which will blend in on the bike without having to be painted.

Be aware that the threads for the brake line are a fine 1mm pitch and if your brake line threads into the end of the cylinder you will need to get a different brake line with a 10mm banjo fitting on the end.  I had a braided stainless one made up locally for about $50 for a total cost that was still less than what a rebuild kit for the crap Magura master cylinder would cost.

It was also found that the line to the reservoir is not the right kind of rubber for DOT4 and needs to be replaced.  That will cost you another couple bucks.

[Martin]

I wasn't really happy with the Chinese boot. So after a bit of research I manage to find a source of better boots. We had trouble with mounting the new boots as they had a habit of sliding off with or without the fitting of a hose clip. I solved the problem by machining a groove in the cylinder for the ridge on the boot to sit in. The boot now fits with or without a hose clip. You can also leave the Chinese boot on or not. However due to the master cylinder being subjected to road spray I also pack the boot with thick silicone grease. This witll greatly reduce the ingress of road crap and should extend the life of the cylinder.
Regards Martin.

Master Cylinder Boot Mod 1.jpg (51.65 kB . 768x432 - viewed 227 times)
Master Cylinder Boot Mod 3.jpg (55.23 kB . 768x432 - viewed 218 times)

[beemuker]

These ms are available on Amazon, a little pricier than getting them from China, but with prime shipping , gets to u in 2 days

Motorcycle Rear Brake Pump Master Cylinder, Hydraulic Pump with Reservoir for Most Motorcycle Dirt Bike ATV

[beemuker]

Got mine bolted up but looking for a source for the rubber boot . Max and EME are out.

[The Mighty Gryphon]

I used the boot that came with it.  I put a cable tie around it to keep it in place on the master cylinder.  Look at the photos I posted.  So far, no problems. 

[beemuker]

Mine didn’t come with an outside boot. Just a flimsy little one before the nut it doesn’t look like it does anything the one on my old master is all torn up.

[Martin]

I chased all over town trying to source boots after the local master cylinder sleeving guy retired. Eventually went to my local friendly brake hose guy and he managed to source some for me for $5.00 Au each. He just needed a picture and relevant dimensions. He might not be able to get them next time.
Regards Martin.