Author Topic: Aftermarket Rear Master Cylinder Replacement Guide $10-$25  (Read 35757 times)

Offline lmiklosy

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #100 on: January 30, 2019, 07:21:00 PM »
Rear MC rebuild update:    I rebuilt my MC using a piece of tough silicone rubber brakeline.  Slid the brakeline on a mandrel chucked in a drill, sanded the contour to a piston seal shape and installed the seal on the piston with some Brembo silicone grease.   This fix cost $0 and works, no leaks since 2017.   My Chinese brake cylinder remains in a box  as a spare.    :twothumbsupp
  • Laguna Beach, CA
  • 1993 K75S-Mystic, 1981 R100RS-RedBaron, 1991 K75S-Marrakesh

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #101 on: March 10, 2019, 05:07:08 PM »
Update as of March 10, 2019:

I have now done the retrofit on a total of four bricks.  Two 1994 K75's with ABS and two K100RS 16V. 

First off, there appears to be two ways the rear brake line is attached to the master cylinder.  Either with a banjo fitting or by screwing a threaded hose end fitting into the end of the master cylinder.  The banjo fitting is the easiest, just use the bolt that comes with the new master cylinder.

The brake line that screws into the end of the master cylinder will raise problems as the Chinese master cylinder has a different thread.  In order to connect the rear brake line you need to get a new one with a banjo fitting so you can use the bolt and washers that come with the new master cylinder.  Another option(that I haven't tried) is to have a new hose made with a fitting with the correct thread.  Either way, you need a new brake line.  If the existing line is original, you probably need a new one anyway.  Even if you are replacing a newer line, you are still ahead of the game with the money you save with the Chinese master cylinder.

Second, Instead of trying to splice the new supply line to the old one, I just go to the auto parts store and buy 3 feet of rubber vacuum line.  A couple inches of 1/4" i.d. clear vinyl tubing from the hardware store is also needed.  Total cost should be under $3.

Take the fabric covered hose off the bike's reservoir.  You need to open the clamp to do it.  Be careful not to break the hose barb on the reservoir.  Push the 1/4" vinyl tube onto the barb, and cut it off even with the end of the barb.  Spread a small drop of liquid  soap on the tube, and push the 5/16" vacuum line over it.  With the tube and hose I get, the fit is so tight no clamp is necessary to hold it together.  Feed the hose down and connect it to the new master cylinder using the clamp that came with the cylinder.

The bonus here is that the 5/16" vacuum line allows the fluid to flow more easily to the master cylinder, making the bleeding and flushing of the rear brake incredibly easy compared to doing it with the old 6mm OEM line.
  • 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
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Offline Martin

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #102 on: August 18, 2019, 09:05:02 PM »
Master cylinder update, over two years on Chinese master cylinder with no problems. I had one as a backup but I kept of having to sell them.  One was mate of a mate and has not reported any problems with his after a year of use. I just ordered another spare $14.05 Au slightly up in price from $13.33Au. However this time I used a different supplier and it comes in black so worth the extra .72 cents. I would never bother going back to an OEM cylinder or a kit. It's a lot easier and quicker to just replace it with another Chicom one.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline lmiklosy

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #103 on: August 19, 2019, 12:37:08 AM »
Same here, piston seal fix still holding no worries. Spare Chinese MC remains in box.   :bmwsmile
  • Laguna Beach, CA
  • 1993 K75S-Mystic, 1981 R100RS-RedBaron, 1991 K75S-Marrakesh

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #104 on: August 19, 2019, 07:13:43 AM »
I've installed them on four bikes now.  Total mileage on the master cylinders is over 20,000 miles with no problems.  The modifications necessary to install them are fairly straightforward to me now and are easier than doing a rebuild(and with a lot better results).

The only thing you need to be careful of is the brake line fitting on the cylinder.  Some of our bikes have the line attached with a banjo fitting, and others have the line threaded into the master cylinder.  You may have to use a new line with a banjo fitting.  No big deal because even with a new line it's still cheaper than buying a rebuild kit.
  • 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 Martin

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  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline tannerman917

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #106 on: September 17, 2019, 11:28:43 PM »
Just mounted up my new RMS... I didn't like how little travel I was getting with the piston threaded directly into the pedal, so I made a second joint for the piston that I'm gonna try.  I rounded off the threads on the end of the stem and drilled out a cap screw with a ball end mill so it rotates freely.  We'll see how it works once I get a new brake line for it.
  • USA
  • '85 K100 RT

Offline sammysnail

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #107 on: October 01, 2019, 10:10:01 AM »

   I am always aware of, and grateful for, all those people who share their knowledge and experience via the internet. In this case I refer particularly to the Young Engineer, Martin, The Mighty Gryphon, and the others who have posted here.

   I try to help when Iím able to, so here are my experiences. Itís a bit of a rambling story but someone might find it useful. Settle back and get comfortable.

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

    I stumbled on the Chinese master cylinder concept when searching Ebay for a BMW rear master cylinder. ďChinese master cylinder, fits pit bike and BMW K100Ē (!!!). Further investigation led me here.

   The path I took involved using a 500mm braided teflon caravan brake line to replace the entire (non-ABS) brake line from the new Chinese master cylinder to the caliper. The line cost about $50 landed. 400mm would have been better, but the 500 was on offer.   Initially I was going to use a 90 degree banjo fitting on the master cylinder, with a female section silver soldered onto it. The banjo bolt was to be cracked to aid in bleeding.

   Then I changed my mind and decided to use a straight connection  that replaced the banjo bolt (nominally m10 x 1.25) with a straight adaptor (M10x1 female to M10x1.25 male), thus coming straight out the back of the master cylinder as per original. If necessary, to aid bleeding the footrest plate can be dismounted and inverted  (as detailed in someoneís post somewhere). The master cylinder arrived and it turned out that they used an odd thread for the banjo bolt. The diameter is  9.75mm, but the pitch is still 1.25mm, so a thread tap was used to open it out to M10x1.25. The banjo bolt came with a couple of Dowty-type washers, which was handy. A Dowty washer is a metal washer that incorporates a rubber o-ring bonded to the inside diameter. When the bolt is tightened the rubber deforms, but only to the thickness of the metal washer. This gives a great seal. Itís a simple and brilliant invention. Dowty was an hydraulics company, and among other things made air over oil suspension for motorcycles (Velocette and Panther come to mind).

   Because the caliper and the new braided line use different flares I made an adaptor to mount the new pipe to the caliper. This consisted of a short length of brake pipe (ex-Holden Commodore)  from the caliper ending in a female fitting (ex-Subaru),turned down and bored to take the pipe, to which was then silver soldered. This pipe was fitted to the caliper and then bent (in the fashion of the original pipe) so that it aimed at the centre of the hole in the  ďCĒ section bracket on the side of the swingarm.  I  bought the M10x1  to M10x1.25 master cylinder adaptor across the counter at the local brake and spare parts place. It was about $5. I used one of the Dowty washers between it and the master cylinder.

   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.... however..... a) Iíd already bought the braided hose and b) this solution is not complex enough.
.
   If youíre in Australia you can use a Holden Commodore rubber front brake line to do the same job. You will need a joiner for the caliper end. This is a m10x1 double ended thingo that is hard to get. Or you could make or fabricate one (two sleeve nuts welded back to back might do it). The other end of this hose is a banjo type fitting.

   The pin-in-a-dimple operation of the master cylinder pushrod seemed rather crude, and like The Mighty Gryphon I considered a heim -type joint (rose joint on the threaded rod and a saddle on the adjuster bolt on the pedal. Move the master cylinder back one hole and brace it with a bracket).

    Then I tried a mock-up of the pin-and-dimple and decided that it was elegantly simple. I decided to put the dimple in the end of the master cylinder threaded rod so that I could retain the original rubber boot. I also put a nylock nut on the threaded rod until it just started to bind on the nylon. This helps to guide the pin into the dimple.

   The master cylinder that I got had mounting holes at 40mm centres, not the advertised 45mm. I had to file one of the holes to make it fit, which was a bonus as I was able to file it at a bit of an angle in order to bring the threaded rod more central to the tit on the end of the brake lever adjusting screw. This is not critical, as the rod can be moved through a fair arc. This rod has a mushroom head which fits into a recess in the end of the piston. A washer fits over the rod and is retained in the bore by a circlip. The rod is thus  spring loaded to the same axis as the bore, but will move from side to side with not too much effort. This assembly makes it easy to remove the rod to work on it (I used a lathe, but thatís probably overkill).

   The 500mm hose was a bit hard to route neatly due to its excessive length, so I made a bracket which hangs down from the front mounting screw which attaches the tragkorb frame to the footrest plate. This makes the hose hang in a loop in front of the final drive, keeps it away from the centre stand, and also reduces the strain placed on the hose where it joins the master cylinder as a result of suspension movement.

   I ran a new 8mm nitrile hose from the original reservoir to the new master cylinder. It was a reasonable fit on the barb of the 6mm reservoir pipe, and tightened up nicely with a hose clamp. Length 300mm, cost with clamps $7 from the local clutch and brake place.

   The system was bled using a vacuum bleeder, a first for me. I can see now why Martin uses a rigid jar with his to catch the fluid. I used a plastic soft drink bottle and just about pulled it inside out with the vacuum. Bleeding was quick and easy and there was no need to invert the footrest plate.

       Case closed.

  • 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 Martin

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #108 on: October 03, 2019, 11:26:55 PM »
BE WARNED I'd be more concerned about the quality of the reservoir hose. I  was investigating a weep coming from just below the adaptor that went between the OEM reservoir hose and the Chinese hose. As I tried to wipe where the leak was occurring the hose just split and dumped the contents of the reservoir on the garage floor. I went to my favorite local brake guy to go and get a new hose and some brake fluid. He had a look at the split hose and said it was definitely not rated for brake fluid and said it was dangerous in this application. He shook his head when I said it had been supplied with the master cylinder.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline Martin

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #109 on: October 04, 2019, 12:57:19 AM »
Just took a couple of pictures of the hose it split just under the hose clip. I was lucky it did it in the workshop and not on the three day ride I'm about to undertake. If it had split on the road it would have covered the rear tyre in brake fluid.
Regards Martin
 [ Invalid Attachment ]
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline sammysnail

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #110 on: October 05, 2019, 08:36:31 AM »
Martin,
 The tubing that came with the master cylinder didn't fill me with confidence, but as I wanted to replace the entire tube, I wasn't using it anyway. I was about to use clear nylex tubing, but then I did a bit of internet research.

It turned out that very few materials are suitable for use with brake fluid. Those that seemed to be suitable were Tygon 2375 (clear), EPDM, Viton, and nitrile. None of them cheap.

The hose that the local place sold me is lined with nitrile rubber. Nitrile is apparently OK with brake fluid at low pressure and temperatures below about 80C. The tubing that I got is 5/16" and red on the outside , marked "EXE MULTIPURPOSE HOSE 5/16" X 50' (300 PSI)"

Time will tell

The following website has a bit of an explaination of the suitable materials:
https://www.pegasusautoracing.com/document.asp?DocID=TECH00147


By the way, something weird happened with my previous post. It was rather long, so I wrote it in a word processor document and then copied and pasted it into the reply window. When I pasted the text the word "p a n n i e r" was automatically replaced by the word "tragkorb", which is the German word for "p a n n i e r". Very strange. Did it again this time, which is why I had to use spaces between the letters.


.
  • 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 The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #111 on: October 05, 2019, 08:55:36 AM »
Thanks for the info on suitable hose materials.

I would caution against using EPDM hose on a bike.  The motor oil, fork oil, and fuel will turn that hose into black goo not unlike what happens to the fuel pump vibration damper.
  • 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 #112 on: October 05, 2019, 08:59:04 AM »
By the way, something weird happened with my previous post. It was rather long, so I wrote it in a word processor document and then copied and pasted it into the reply window. When I pasted the text the word "p a n n i e r" was automatically replaced by the word "tragkorb", which is the German word for "p a n n i e r". Very strange.
You're a stranger in a strange land, sammy. That's not strange here. Read enough posts and you'll be illuminated. Go with the flow. Tragkorb fairly rolls off the glottis. :-)

Thanks for your perspective concerning the Chicom brake cylinder installation.
  • 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.
https://tinyurl.com/RillRider

Offline E30_Crazy

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #113 on: October 05, 2019, 09:05:28 AM »


By the way, something weird happened with my previous post. It was rather long, so I wrote it in a word processor document and then copied and pasted it into the reply window. When I pasted the text the word "p a n n i e r" was automatically replaced by the word "tragkorb", which is the German word for "p a n n i e r". Very strange. Did it again this time, which is why I had to use spaces between the letters.


I believe it's a code built into the reply function of the website (not too sure on the technical terms). I have seen it change a couple different words. Alternator is, or was one. The Cafe/scrambler/street/etc builders often get hemmed up on "c l i p - o n s" turning into "strap-on".

That, or a Moderator/Admin frequently searches for terms and changes them manually.
  • Newport News, Virginia
  • '85 K100, '93 K1100RS
God created Crew Chiefs so Fighter Pilots could have heroes, too.

Offline Laitch

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #114 on: October 05, 2019, 09:16:07 AM »
I believe it's a code built into the reply function of the website (not too sure on the technical terms).
Substitution Cipher. :-)
  • 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.
https://tinyurl.com/RillRider

Offline billday

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #115 on: October 05, 2019, 09:36:45 AM »
I'm trying to make sense of this thought process:

  • Question: How can I save a few bucks?
  • Answer: By buying the cheapest brake parts I can find.



  • New York State, USA 10977
  • 1985 K100

Offline Laitch

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #116 on: October 05, 2019, 10:00:35 AM »
I'm trying to make sense of this thought process: . . .
It leaves more money for the memorial service and heirs. Every penny counts when intending to use a caisson flanked by fan dancers for conveying the cremains.
  • 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.
https://tinyurl.com/RillRider

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #117 on: October 05, 2019, 05:43:39 PM »
Really, I enjoy living on the edge of disaster.
  • 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 sammysnail

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #118 on: October 05, 2019, 06:54:24 PM »

Laitch, I am an egg.
  • 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 sammysnail

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #119 on: October 05, 2019, 06:58:47 PM »

You're a stranger in a strange land, 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 stokester

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #120 on: October 05, 2019, 07:29:07 PM »
I'm trying to make sense of this thought process:

  • Question: How can I save a few bucks?
  • Answer: By buying the cheapest brake parts I can find.
Thought maybe I was the only one.  The original lasted 25 years or more and costs about $200.
  • Yorktown, Virginia
  • '94 K75S Dakar Yellow - '93 K75S Seiden Blau - '91 R100RT Bermuda Blue- '78 R100S Smoke Red

Offline Laitch

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #121 on: October 05, 2019, 07:40:51 PM »
Laitch, I am an egg.
Your writing isn't reflecting that. Are you using a ghostwriter?
  • 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.
https://tinyurl.com/RillRider

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #122 on: October 05, 2019, 07:49:23 PM »
Thought maybe I was the only one.  The original lasted 25 years or more and costs about $200.

Actually, $269, and will probably be about $290 when the next price bump comes this winter.

Personally, I can't justify that kind of price for a couple die castings, two screw machine parts, and a couple rubber and plastic bits.  There is nothing magical in making a master cylinder.
  • 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 Martin

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #123 on: October 07, 2019, 02:54:53 AM »
Just got home from our 3 day ride to the Cane Toad Rally, one K1100, three K100 RS's, one K100 LT, one K75 RT and one K75 covered a fair bit of northern NSW. Total problems with the K's one RS mirror housing fell off and I blew a indicator dash globe that was fitted OEM in 1992. No problems with the new brake reservoir hose which is at least four times thicker and is rated for brake fluid and marked as such. In order to use the new hose which didn't fit the OEM reservoir I temporarily used the Chicom one until I get around working out how to fit the OEM one.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline sammysnail

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #124 on: October 07, 2019, 09:40:05 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.
  • 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).

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