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1Back to top Go down   Rear brake question 85 K100RS Empty Rear brake question 85 K100RS on Mon Apr 20, 2015 11:07 pm

diversity48

diversity48
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After bleeding the front brakes succesfully, I tried bleeding the rear. Seems that I have a bad Master cylinder, here is what it does: Removed most of the fluid from the reservoir, filled it with fresh fluid, and proceeded to drain the caliper using a "Mity-Vac" which I have used on other bikes and on the front calipers with complete success. Only could get a few drops into the canister of the mity-vac at first, and after pumping it several times, ended up with only a very small amount of fluid there...possibly 1/2" or so in the bottom of the canister. Tried bleeding the old fashioned way...pumping the pedal, and after MANY pumps, probably 100-150 or so, finally got the rear brake to function, but just barely. Now I have very little rear brake power. After reading related posts on the subject, I'm thinking that it may be the master cylinder. With these symptoms, what would your opinion be? I have not taken the caliper off, but I'm thinking that even with leaky seals, the caliper should at least be kicking out some fluid from my efforts. Just need some educated guesses here...what do you think?

    

2Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Mon Apr 20, 2015 11:35 pm

KafeRacer

KafeRacer
Silver member
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I have a rear master if you need one $50 usd delivered.

Cheers.


__________________________________________________
Dave Evans
'Kafe Racer'
kaferacer.wordpress.com

1990 K75RT
1985 K100 Kafe Racer
http://kaferacer.wordpress.com
    

3Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Mon Apr 20, 2015 11:42 pm

Point-Seven-five

Point-Seven-five
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Are you seeing the fluid level in the reservoir going down?  If not, there may be a blockage in the supply from the reservoir to the master cylinder.

Is there fluid leaking out under the boot where the pedal pushes on the master cylinder piston.  If yes, the piston seals are bad and you're pissing the fluid out on the floor. 

Can you disconnect the brake line at the caliper and get fluid when the master cylinder is pumped?  If yes, you may have a blocked bleeder.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

4Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 21, 2015 12:31 am

diversity48

diversity48
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No leaking beneath the bike ... it would be obvious if it was leaking there. I pumped the hell out of the pedal and the reservoir on the bike remained at or very near the same level. Normally, with the bleeder system I have, it supplies enough "suck" to deplete the reservoir rather quickly. Not so in this case.  The reason I'm suspicious of the master cylinder is that no, or at least very little fluid seems to be getting back to the caliper. It went from "no rear brake" to "very little" rear brake.

    

5Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 21, 2015 12:34 am

diversity48

diversity48
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Oops...NO, I haven't tried disconnecting the hose at the caliper...good thought though...at least worth a try. Also could be the blocked bleeder, but if it were, wouldn't the brakes at least function?

    

6Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 21, 2015 1:50 am

brickrider

brickrider
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I replaced the line on the rear brake last year thinking a stainless steel one would firm up the pedal.  No joy.
My bike is a 1985 K100RS too.  It has only about 40K on the clock, but that spongy rear brake has me wondering if the m/c isn't K-putt.  Does BMW sell rebuild kits for the master cylinder of the rear brake?

    

7Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 21, 2015 6:04 am

92KK 84WW Olaf

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@brickrider wrote:I replaced the line on the rear brake last year thinking a stainless steel one would firm up the pedal.  No joy.
My bike is a 1985 K100RS too.  It has only about 40K on the clock, but that spongy rear brake has me wondering if the m/c isn't K-putt.  Does BMW sell rebuild kits for the master cylinder of the rear brake?

I had same issue with my 92 K100LT rear brake. We got a rebuild kit for the master cylinder and it did the business. also did the 84 K100RT last April. Rebuild kits used in both cases but the RT needed a replacement reservoir [cheap] and also needed the pistons and caliper seals sorted. Pistons were sticky giving no real braking pressure.


__________________________________________________
1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011171 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 37,190 miles
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles.
    

8Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Wed Apr 22, 2015 1:01 pm

Dai

Dai
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I suspect a rebuild will be necessary. From your description I'm pretty sure that the feed hole in the m/c is blocked, so no (or very little) fluid is getting into the brake line. A blocked return hole will give similar symptoms.

What doesn't help is that BMW have a rubber block between the brake pedal and the m/c piston. This softens up the feel still further. My guess is they designed it in there to make locking up the rear wheel less of a problem. I threw mine away and replaced it with a length of stainless steel shaped to mimic the plate on the end of the rubber block. Much more feel and feedback.

While you're down there, pull the rear brake caliper off, remove the pads and scrub the guaranteed pile of crap out from the middle of the caliper. That will definately help with the braking. The dirt that gets in there can prevent the pads from moving smoothly; if it's bad enough I've seen it stopping the pads moving at all.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

9Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Thu Apr 23, 2015 11:08 am

diversity48

diversity48
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Guess I never really thought the caliper would be the suspect, but at least a :going through" on it would eliminate that. Gonna try that (hopefully today) first and see how far I get. Next would be either a new master or rebuilding the old one. Found a new one at bmw2valve.com for $169 that may be the ticket although more than I wanted to spend :-(...starting with the caliper first tho without pulling the trigger on the new m/c.

    

10Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Thu Apr 23, 2015 2:54 pm

brickrider

brickrider
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Personally, I'm inclined to go to MotoBins for a rebuild kit.  My bike seems to have been pretty well maintained by the previous owner(s) so I doubt the bore of the master cylinder is so pitted that a rebuild won't restore proper action.
And what's this about a rubber bit to soften the feel of the pedal?!  I must have a closer look down there.  That sure sounds like a daft idea to me, but then what do I know as compared to the BMW engineers?  Shocked

    

11Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Thu Apr 23, 2015 5:03 pm

sidecar paul

sidecar paul
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Rear brake question 85 K100RS Ssl10911

That black bit is the rubber part, Brickie......I tried replacing it with a solid stainless part, but I didn't like it (no 'feel' at all) so I added a 2.5 mm thick ring on the rubber to reduce the amount of squish. Works for me.

Rear brake question 85 K100RS Ssl10912

Paul.


__________________________________________________
'84 K100RS (0014643) (owned since '85), 86 K100RS (0018891) with Martello sidecar (built as an outfit in '88),
'51 Vincent (since '67),'72 Montesa Cota (from new), '87 Honda RS125R NF4 (bought 2015) 
....No CARS never ever!
    

12Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Thu Apr 23, 2015 5:54 pm

Dai

Dai
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One thing to bear in mind when overhauling Brembo master cylinders (and someone here is really going to argue with me over this!); two overhauls and that's your lot. Shocked

Okay, so that's the factory statement  Very Happy

Experience shows something different but there is a limit. When you've got the piston out, hold the body up to the light and check the insides very carefully. If it's nice and shiney and has wide silver streaks in there, introduce it to the bin. The surface anodising has worn off and the chances are even a new seal will either not work or will not work for very long. 

Yes, I have fallen foul of this. The symptoms are an overhauled system that will bleed well but will not hold pressure. It happened to me about fifteen years ago and I ended up having a very nice chat with a Brembo rep who gave me the 'two overhauls' information. Unfortunately I had to buy my own replacement master cylinder Sad


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

13Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Thu Apr 23, 2015 6:24 pm

brickrider

brickrider
Life time member
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I've heard it said by some of the wise men in the airhead community that even one rebuild on those front brake master cylinders is a waste of time and money. I understood the reason was pitting of the bore; never thought of the possibility of them being anodized or otherwise finished in any way. 
I like the idea of a ring to limit the expansion of the rubber bit in the m/c.  I can see that having a lathe in the shed is better than a very good thing Smile  ... but then I'd have to learn how to use it Razz



Last edited by brickrider on Thu Apr 23, 2015 6:25 pm; edited 1 time in total (Reason for editing : grammar)

    

14Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Thu Apr 23, 2015 6:24 pm

92KK 84WW Olaf

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I have to agree with this.

Not a surprise I hope.


__________________________________________________
1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011171 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 37,190 miles
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles.
    

15Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Thu Apr 23, 2015 6:44 pm

Dai

Dai
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@brickrider wrote:I've heard it said by some of the wise men in the airhead community that even one rebuild on those front brake master cylinders is a waste of time and money. I understood the reason was pitting of the bore; never thought of the possibility of them being anodized or otherwise finished in any way.
I have to utterly disagree with that sentiment. AFAIK the only thing that could cause pitting would be poor maintenance allowing sh!t to get in and/or not changing the fluid often enough. In almost thirty years of overhauling Brembo master cylinders on Guzzis, Laverdas and BMWs I have seen two that were visually worn beyond use (including the one that caught me out) but never one that was pitted. As I'm only a home mechanic and not a bike pro, I'd guess I've done perhaps 30-40 master cylinders front and rear and maybe 60-80 calipers of the P08/F08 type.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

16Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 28, 2015 11:05 am

SteveK1

SteveK1
Silver member
Silver member
@Dai wrote:
@brickrider wrote:I've heard it said by some of the wise men in the airhead community that even one rebuild on those front brake master cylinders is a waste of time and money. I understood the reason was pitting of the bore; never thought of the possibility of them being anodized or otherwise finished in any way.
I have to utterly disagree with that sentiment. AFAIK the only thing that could cause pitting would be poor maintenance allowing sh!t to get in and/or not changing the fluid often enough. In almost thirty years of overhauling Brembo master cylinders on Guzzis, Laverdas and BMWs I have seen two that were visually worn beyond use (including the one that caught me out) but never one that was pitted. As I'm only a home mechanic and not a bike pro, I'd guess I've done perhaps 30-40 master cylinders front and rear and maybe 60-80 calipers of the P08/F08 type.

OK, so from what you are saying I cannot overhaul my front master-cylinder?  I have a 16 valve K1 and I think it has an all-aluminum 20 mm MC.  I rebuilt it previously with new piston and seals, honed the bore.  Several months later it seized up the front calipers and pitched me off.  Thinking it may have been not cleaned adequately I have re-honned it and cleaned it in a sonication bath.  I was about to reassemble it.  Are you saying I would be unwise to try using this and would do better to source a new MC body? Could one re-anodize the bore?



Last edited by SteveK1 on Tue Apr 28, 2015 11:11 am; edited 1 time in total (Reason for editing : spelling)


__________________________________________________
BMW K1 1990
BMW R60 /5 1973
BMW R100R 1992
Honda CB360T 1975
    

17Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 28, 2015 2:46 pm

Dai

Dai
Life time member
Life time member
Personally I'd be looking for a replacement master cylinder.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

18Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 28, 2015 3:34 pm

SteveK1

SteveK1
Silver member
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@Dai wrote:Personally I'd be looking for a replacement master cylinder.
Personally? Why? Because of the all aluminum versus steel cylinder? More information please. 

Reading in the thread, it looked to me like the MC I have should be repairable.
 
https://www.k100-forum.com/t1301-brake-seizure


__________________________________________________
BMW K1 1990
BMW R60 /5 1973
BMW R100R 1992
Honda CB360T 1975
    

19Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Tue Apr 28, 2015 6:39 pm

Crazy Frog

Crazy Frog
admin
admin
Time to give my one cent of advise and experience..

Have you tried to play with the adjustor screw on the MC (This is the link between the pedal and the MC- #12 on the first picture- and screw #3 on the second one)
If the travel of the piston is limited, the piston will never return to the position where the filling hole is cleared to let more brake fluid re-enter the MC.
It's worth playing with the adjustments before replacing the unit.
Rear brake question 85 K100RS Mc10
Rear brake question 85 K100RS Mc210


__________________________________________________
Rear brake question 85 K100RS Frog15Rear brake question 85 K100RS Logo2101986 k75, 1985 K100rt, 1985 K100rt/EML GT2 sidecar, 1999 K1200lt/Hannigan Astro Sport sidecar.
    

20Back to top Go down   Rear brake question 85 K100RS Empty Re: Rear brake question 85 K100RS on Wed Apr 29, 2015 1:22 pm

Dai

Dai
Life time member
Life time member
@SteveK1 wrote:
@Dai wrote:Personally I'd be looking for a replacement master cylinder.
Personally? Why? Because of the all aluminum versus steel cylinder? More information please. 

Reading in the thread, it looked to me like the MC I have should be repairable.
 
https://www.k100-forum.com/t1301-brake-seizure
As you say, it's been honed twice, so I wouldn't trust a new seal kit to make a good seal in there. Perhaps I'm being too cautious but experience is a hard taskmaster and I'd rather be absolutely sure of my brakes. You could try an overhaul kit and find it will work perfectly; I'd also check the two holes very carefully when you have it apart.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

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