Author Topic: Aftermarket Rear Master Cylinder Replacement Guide $10-$25  (Read 25109 times)

Offline YoungEngineer

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  • Posts: 109
Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #50 on: February 15, 2017, 09:22:06 AM »
Glad to see this has worked out for you. Yes the bleeding is a pain in the ass, as you say this is worsened by the inverted reservoir tube. I found that disconnecting the brake line from the caliper and sucking on it aided bleeding, but be prepared for endless jokes from passers by if doing this in a public place.


Mine to date is still working out just fine, the only issue I noticed was that my caliper was sticking, but after cleaning/greasing the pistons its now retracting correctly.




If I come up with any other El-cheapo mod's Il be sure to share them.

  • London
  • K1100LT

Offline Martin

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #51 on: February 28, 2017, 12:37:22 AM »

While I'm waiting for the boat from China to arrive with the new master cylinder, decided to clean and repaint both foot peg plates. Removed  and painted the left one. While the left was drying removed the right hand one, and noticed the spring that attaches to the brake lever had gone AWOL. Had a look at my Haynes manual and it shows it attaching to a bolt with two sleeves on it. I can't find the pictured bolt and somewhere in the dark hazy recesses of my mind I think it attaches to the pin that the centre stand springs attach to. It is not a big deal as I can make something to attach it to but I might as well try to get it right. So where does it go, I had a look at Gryphs conversion but it does not show.
Regards a slightly perplexed Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #52 on: February 28, 2017, 12:46:15 AM »
I don't have a photo, but one end of the spring connects to that flat bit connected to the threaded pushrod on the pedal. 

The other end hooks over the pin that the upper end of the two centerstand springs attach to.  There is a groove between the hooks of the big springs that the pedal return spring goes into.  The groove may be obscured by grease and muck.

Hope this makes sense.
  • 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 #53 on: February 28, 2017, 01:34:26 AM »

Many thanks Gryph that's what I thought. I shouldn't have looked at the Haynes which totally threw me. I've got wait till tomorrow to assemble it, paint still drying. When I get the new  cylinder I've come to the conclusion that I'll have to get a new hose, it's too much stuffing around to make an adapter. I've also decided to make a stainless steel shield to hide the hose.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #54 on: February 28, 2017, 07:46:13 AM »
Martin, I wouldn't worry about hiding that hose from the reservoir.  It's not visible when you are closer than about 5 feet from the bike even when you are looking for it.  It isn't really in any danger of dragging because it is far enough inboard and the center stand protects it from bottoming out on things like curbs.

Update on my installation: 

I now have about 150 miles on the bike since installing the new master cylinder.  Rear brake works better than it ever did since I got the bike 28,000 miles ago.  It is now possible to completely stop the bike at low speed with just the rear brake which is something the bike never did before.   I don't even think about it anymore.  Operation and pedal feel is totally transparent.

So far it looks like a very well spent $20.  If I ever meet the Young Engineer I will buy him a drink, probably several.   

Screw BMW.
  • 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 YoungEngineer

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  • Posts: 109
Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #55 on: February 28, 2017, 09:56:46 AM »
I'm glad it worked out for you. I have also experienced good results with my setup. So far no issues on my end even with the notorious bad British weather.
  • London
  • K1100LT

Offline lmiklosy

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  • Posts: 67
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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #56 on: March 08, 2017, 01:42:10 AM »
There are on-line stores with more rebuild parts but none offer dimensional data. Here's one example: http://stores.ebay.com/ClassicCycleParts-Online/_i.html?_nkw=master+cylinder&submit=Search&LH_TitleDesc=1&_sid=24435484   

Another: http://www.motocrossgiant.com/ProductDetails.asp?ProductCode=232%2D4153wps

The rebuild kit runs about US $90  here's one installed:  https://www.motobrick.com/index.php/topic,8790.msg68520.html#msg68520
  • Laguna Beach, CA
  • 1993 K75S-Mystic, 1981 R100RS-RedBaron, 1991 K75S-Marrakesh

Offline lmiklosy

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #57 on: March 08, 2017, 06:01:36 PM »
 :deal:    A couple fine points.  BMW microfiche shows the MC for K75 is Magura not Brembo while BMW Boneyard explains a lack of Magura parts due to a lawsuit with BMW. I find Magura MC rebuilds in 9mm and 10.5 mm sizes but not 12, 13.   Martin reminisces over replacement piston cups in years gone by but no longer available  :dunno2:   what size are the MC bores (alternatively pistons) for those of us wishing to replace cups?  K75 microfiche specifies 12mm for ABS and 13mm for non-ABS MC bores, what are the exact measures? Somebody with a micrometer could measure their OEM part and post here for reference.   Here are usable cups on a KTM rebuild kit, piston measures 12.9mm:  http://www.ebay.com/itm/311421694121?ul_noapp=true
Getting cups off an assembled piston intact is worthy of another thread tho.   Hot water? Sauna and coconut oil?
  • Laguna Beach, CA
  • 1993 K75S-Mystic, 1981 R100RS-RedBaron, 1991 K75S-Marrakesh

Offline Laitch

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #58 on: March 08, 2017, 06:37:20 PM »
I find Magura MC rebuilds in 9mm and 10.5 mm sizes but not 12, 13.   what size are the MC bores (alternatively pistons) for those of us wishing to replace cups? 
Max BMW has Magura rebuild kits for 12mm and 13mm.

  • 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.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline lmiklosy

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #59 on: March 08, 2017, 07:55:58 PM »
Ariete is an Italian manufacturer of motorcycle plastic bits, heed their warning "Don't use Chinese junk on your classic bike"  :nono
  • 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 #60 on: March 09, 2017, 03:00:51 PM »
The Chinese would have to lower the bar an awful lot to be as bad as the stupidly priced BMW/Magura master cylinders and rebuild kits I have had to deal with.

At least with the Chinese stuff I'm not grossly overpaying for what I get.
  • 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 #61 on: March 09, 2017, 10:12:24 PM »

Received my master cylinder on Wednesday full price including the slow boat from China $14.00 Au, which means I can buy 21.42 Chinese ones to one OEM one. A friend of a friend made me up a new hose for the exorbitant price of $10.00 in stainless braid.  He also threw in a new rubber boot to go over the end of the cylinder and is going to put me onto a local source for  boots and cups. I bought a new 8mm x 1.25 bolt to replace the OEM brake pedal stop which I will machine a recess into the head in order to keep the brake cylinder rod from moving off the stop. While I am at it I will make a reducer to go between the OEM reservoir hose and the new reservoir hose. I will keep and store my OEM hose and and still functioning cylinder as a spare. I will post pictures of the conversion when I am finished. With the savings of $276.00 I am wondering how many profiteroles I can buy.  :lets-eat:
Regards Martin
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline Elipten

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #62 on: March 09, 2017, 10:21:18 PM »
Martin

Keep a drawing with the dimensions that way others can get their parts machined.


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  • San Antonio, TX
  • 1990 K75RT

Offline Martin

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #63 on: March 09, 2017, 10:29:13 PM »

I am just machining them because I can, you could do the same thing with a drill press. It is just the reverse of what Gryph did, he put a recess in the end of the push rod. But I will take photos of the process, however my lathe is old and very sensitive and it does not tolerate being laughed  at. I think it was once owned by Noah.  :dunno
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #64 on: March 09, 2017, 11:06:41 PM »
Martin, I didn't even use a drill press.  I clamped the master cylinder's threaded rod in a pair of vise grips which I then clamped in the vise on my carpenter's workbench(I'm a woodworker, not a machinist) and went at it with a portable electric drill.  The fanciest thing I used was a center guide from QuikCenter to help center the pilot hole in the end of the threaded rod.

  • 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 #65 on: March 10, 2017, 01:46:23 AM »

Gryph I was lucky about 30 years ago I picked up my lathe from a Harley workshop that had gone broke for $300.00. Made back the $300.00 the next week making stub axles for the local Go Kart hire business run by a mate.
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 #66 on: March 10, 2017, 02:45:16 AM »

Elipten as requested a variation on Gryphs method. I cut down the master cylinder pushrod shaft so that 8 mm showing after the nut and carefully removed two threads with a pencil burr grinder. The new stop bolt was cut down to 30 mm, the head thickness was 5.2 mm and the recess was cut in 3 mm. The thin lock nut for the stop bolt was salvaged of the new pushrod shaft. The whole thing was then mounted on to a spare foot peg plate that I borrowed of a mate. When I mount it on the bike I will use flat and spring washer and paint it black. I'll also put on the master cylinder boot I scored over the new boot. Apparently the OEM reservoir hose diameter is different than the new one so a reducer will have to be found or made, Gryph came up with a solution for that problem. If I can't buy one I'll probably make one from aluminium stock in the lathe. Link for 6mm to 8mm barb reducer  http://www.ebay.com.au/itm/Plastic-Barbed-Connector-Pipe-Hose-Joiner-Tubing-Fittings-Air-Fuel-Water-Petrol-/ The new hose total length is 300mm, the guy that made it has contacts for master cylinder cups and boots so I'll definitely be giving him another visit.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline Elipten

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #67 on: March 10, 2017, 07:05:42 AM »
Thanks Martin.  I am sure others will find this helpful


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  • San Antonio, TX
  • 1990 K75RT

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #68 on: March 10, 2017, 09:35:14 AM »
Nice job Martin!  I had the opportunity to buy an Atlas Lathe at a garage sale for $100 about 40 years ago, but at the time I was living in an apartment.  I still regret not being able to get it. 

Are you going to put a boot over the joint between the master cylinder rod and the push rod on the pedal?  I wanted to keep as much dirt and water out of the contact area as possible to protect the lube that I had put on the contact point between the two rods.  The stock boot works pretty well for that.
  • 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 #69 on: March 10, 2017, 02:27:20 PM »

Gryph the OEM  boot won't work with my mod, but the one I got given might. I am going to see the hose guy again Monday or Tuesday and find out exactly where the mob that supplies boots and cups are located. I am also going to chase up a local supplier of the 6mm to 8mm barb hose reducers, although as I posted they can be had online for under $5.00. Today I am heading up in them thar hills to a motorcycle swap meet at Maleny yee har. Was going to go up in my mates ute due to the fact he still hasn't got one of the herd going. But I am now going to ride up due to the fact my lovely wife has decided to share her cold with me. I will post more on this as I get more information. And a thank you to the Young Engineer for the idea, if he's ever in OZ I will bake him a Koala pie. :riding:
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 #70 on: March 10, 2017, 03:45:26 PM »
This might be the Magura repair kit for a K75S (ABS 12mm non 13mm) but the page won't display.  :nono Magura isn't talkin'.
  • 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 #71 on: March 10, 2017, 03:56:37 PM »
From now on rebuild is not an option for my rear master cylinders.  For $12 and two hours in the garage I can't even start to waste my time thinking about BMW or Magura.
  • 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 #72 on: March 10, 2017, 04:06:26 PM »

Same here I won't bother with OEM will only keep it as a spare. But in saying that might just order another Chinese one to keep in stock. I did research the Chinese ones and couldn't find anything negative other than sight glass failures, and seeing as we don't have a sight glass I'm not worried.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline The Mighty Gryphon

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Re: Aftermarket Rear Master Cylinder Replacement Guide $10-$25
« Reply #73 on: March 10, 2017, 04:26:19 PM »
A master cylinder is not rocket science.  A couple aluminum alloy die castings, a few screw machine parts and some rubber bits.  Could be made in the USA for 25-30 bucks. 

$255 is beyond delusional.
  • 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 #74 on: March 11, 2017, 04:22:20 AM »

I used to race Go Karts and have made master cylinders from scratch as Gryph stated we are not dealing with rocket science. I had an epic fail today machining up the 6mm to 8mm reducer, on the last bit the tool caught and it tore itself apart. Two hours down the drain will try again tomorrow with a smaller bore size.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

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