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Author Topic: Intermittent Front Brake Failure  (Read 430 times)

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Offline Max Wedge

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Intermittent Front Brake Failure
« on: May 15, 2019, 12:14:21 PM »
 Hey gang, been bouncing ideas around, anybody have any good theories?

Facts: 2010 GS, with 85,000 miles. I have owned it since new, serviced by the book. On the ride home today on the highway, traffic slowed, and I pulled the front brake, I got no resistance, no braking, nothing. I down shifted and applied the rear brake and slowed. A second pull of the front brake felt normal. Checking it the next few miles, a tug on the lever got me braking. Not knowing if I had a leak int he system, I refrained from braking for 15 minutes. Next time I pulled it, same thing, no brakes, no lever resistance. Pulled it a second time and it was back. Rode for the next 50 minutes, doing light braking occasionally to see if it was still there. It was.

When I got to my sub, I did some hard front brake checks, and it felt normal. Visual inspection shows no leaks on any line, fluid level front and rear are full. Pad material is good. Caliper bolts are torqued correctly. Discs are straight and true, no visible runout. Applying front brake while turning rear wheel yields nothing until I turn the ignition on, then applying the front brake while turning rear wheel causes a little vibration and flutter noise as the rear caliper actuates (linked brakes). Bike has never been crashed. No warning lights. No issues or major repairs. Just solid reliable (until now) transportation.

Any ideas? Next step is I take it to the dealer, say "It all works fine now, try to find out what happened. Here is my left arm and leg." Needless to say, I am hesitant to ride it to work in commuting traffic until I know what is wrong. Really harshes the ride.

I have done some further tests, will now look for bad wheel bearings (6&12 check), and that the disc are actually floating on the buttons and not hanging up and pushing the pads back.
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Re: Intermittent Front Brake Failure
« Reply #1 on: May 15, 2019, 01:09:10 PM »
I’d be looking hard at the front brake(s) master cylinder at the lever.  It’s probably just starting to pass fluid by the seals.  First pull rotates the seals, the second captures the fluid properly.
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Re: Intermittent Front Brake Failure
« Reply #2 on: May 15, 2019, 01:25:34 PM »
Sleazy has it.
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Offline Virginian

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Re: Intermittent Front Brake Failure
« Reply #3 on: May 15, 2019, 01:34:38 PM »
I’d be looking hard at the front brake(s) master cylinder at the lever.  It’s probably just starting to pass fluid by the seals.  First pull rotates the seals, the second captures the fluid properly.

I concur, I believe he is spot on.
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Re: Intermittent Front Brake Failure
« Reply #4 on: May 15, 2019, 02:14:00 PM »
I agree. It may also have some “crud” in the master cylinder preventing fluid from getting to the piston. My manga used to have that problem. Never did figure out what caused it?
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Offline Max Wedge

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Re: Intermittent Front Brake Failure
« Reply #5 on: May 15, 2019, 06:50:19 PM »
I like it, but why would it only do that intermittently? And so drastic when it goes? But fine otherwise?
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Offline maddjack

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Re: Intermittent Front Brake Failure
« Reply #6 on: May 15, 2019, 07:10:18 PM »
It takes time to bleed down thru the shaky seal.After you two pump it t stays pumped up for a while and slowly bleeds past the seal till you have nothing at the initial lever pull.Same as car brakes ,( pre abs )  you could pump the pedal up by repeated stepping on it.
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Re: Intermittent Front Brake Failure
« Reply #7 on: May 16, 2019, 06:18:22 AM »
Yep. I'm all in on master cylinder.
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Offline R Doug

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Re: Intermittent Front Brake Failure
« Reply #8 on: May 16, 2019, 06:42:38 AM »
Good news!  It's a 2010 and not a pre 2007 with servo brakes!

Best of luck w/ finding the issue, brakes are important. 
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Re: Intermittent Front Brake Failure
« Reply #9 on: May 16, 2019, 06:49:37 AM »
If you don't spot any obvious leaks.  Flush the brake system and see if it comes back.  Sounds to me like an air bubble in the system.  If you know it's flushed clean and still comes back, something about the master cylinder isn't working properly.
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Offline Max Wedge

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Re: Intermittent Front Brake Failure
« Reply #10 on: May 16, 2019, 11:48:01 AM »
It takes time to bleed down thru the shaky seal.After you two pump it t stays pumped up for a while and slowly bleeds past the seal till you have nothing at the initial lever pull.Same as car brakes ,( pre abs )  you could pump the pedal up by repeated stepping on it.

I'm with you, but wouldn't it be somewhat consistent? It can sit for days in the garage, no problem, but 10 minutes on the road, and then not again until 50 minutes on the road?  I can see vibrations causing it, but i would think that it would be every ten minutes or every 20. And not doing it at all when parked?

Just thinking, because i'm work and its at home.

Any sure way to test it before I tear it apart?
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Re: Intermittent Front Brake Failure
« Reply #11 on: May 16, 2019, 12:15:43 PM »
It takes time to bleed down thru the shaky seal.After you two pump it t stays pumped up for a while and slowly bleeds past the seal till you have nothing at the initial lever pull.Same as car brakes ,( pre abs )  you could pump the pedal up by repeated stepping on it.

I'm with you, but wouldn't it be somewhat consistent? It can sit for days in the garage, no problem, but 10 minutes on the road, and then not again until 50 minutes on the road?  I can see vibrations causing it, but i would think that it would be every ten minutes or every 20. And not doing it at all when parked?

Just thinking, because i'm work and its at home.

Any sure way to test it before I tear it apart?


I hate intermittent problems.

No real way to test a master cylinder.  They're the worst to diagnose and you didn't say it was ongoing issue.  Your post read like this was the first time it happened.  You've got a butt load of miles on that GS, right?  Not Ed type, but over 65k?


If you rebuild it, there's an easy way to bleed it out.  Bench bleed the master cylinder (google), then install.  Set the handlebars so the master cylinder is at the highest point you can with the cover off and lightly actuate the brake lever to see the bubbles leave the bypass hole in the reservoir.  Once it stops bubbling, pause for a few minutes and do it again.  It'll get 99% of the air out.  Put the cover back on, push the bars to lock and do it a few more times.  I did it many times for a safety recall on Harleys.
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Re: Intermittent Front Brake Failure
« Reply #12 on: May 16, 2019, 12:40:52 PM »
Frickin' scary. Hope you get it sorted.
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Re: Intermittent Front Brake Failure
« Reply #13 on: May 16, 2019, 01:31:42 PM »
It takes time to bleed down thru the shaky seal.After you two pump it t stays pumped up for a while and slowly bleeds past the seal till you have nothing at the initial lever pull.Same as car brakes ,( pre abs )  you could pump the pedal up by repeated stepping on it.

I'm with you, but wouldn't it be somewhat consistent? It can sit for days in the garage, no problem, but 10 minutes on the road, and then not again until 50 minutes on the road?  I can see vibrations causing it, but i would think that it would be every ten minutes or every 20. And not doing it at all when parked?

Just thinking, because i'm work and its at home.

Any sure way to test it before I tear it apart?

It's the master cylinder. Any... EVERY... other option would produce a leak. The master cylinder's push cup is worn, hardened, maybe even got a slight crack in it. Double-pumping it pushes enough fluid to give you some resistance; it's just a flow issue. you're trying to push more on the second pump than will push through/past any gaps that have *kinda* reset on movement of the cup/piston.

It's the master cylinder. I can give you probably 1100 anecdotal experiences where brake lever/pedal went down and in 100% of the cases where a leak wasn't present, it was the master cylinder. Proportioning valves on cars don't do this. ABS systems on cars and bikes don't do this. Leaking things don't do this.

It's the master cylinder.

I turned a wrench from '81 to '91 professionally, and as a side gig since then on everything with wheels and an engine.

It's the master cylinder.

Don't get stuck in analysis paralysis. Just replace it.

PS... it's the master cylinder.
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Re: Intermittent Front Brake Failure
« Reply #14 on: May 16, 2019, 01:42:35 PM »
I'm thinking it could be the master cylinder.....
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Re: Intermittent Front Brake Failure
« Reply #15 on: May 16, 2019, 02:16:15 PM »
Per the forum rules, the inquiry is correctly answered in the first response.....then it's all downhill.

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Re: Intermittent Front Brake Failure
« Reply #16 on: May 16, 2019, 05:28:39 PM »
Per the forum rules, the inquiry is correctly answered in the first response.....then it's all downhill.

Is it still Ed's birthday?

LOL  :clap: :clap: :clap:
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Re: Intermittent Front Brake Failure
« Reply #17 on: May 16, 2019, 05:48:56 PM »
It takes time to bleed down thru the shaky seal.After you two pump it t stays pumped up for a while and slowly bleeds past the seal till you have nothing at the initial lever pull.Same as car brakes ,( pre abs )  you could pump the pedal up by repeated stepping on it.

I'm with you, but wouldn't it be somewhat consistent? It can sit for days in the garage, no problem, but 10 minutes on the road, and then not again until 50 minutes on the road?  I can see vibrations causing it, but i would think that it would be every ten minutes or every 20. And not doing it at all when parked?

Just thinking, because i'm work and its at home.

Any sure way to test it before I tear it apart?

It's the master cylinder. Any... EVERY... other option would produce a leak. The master cylinder's push cup is worn, hardened, maybe even got a slight crack in it. Double-pumping it pushes enough fluid to give you some resistance; it's just a flow issue. you're trying to push more on the second pump than will push through/past any gaps that have *kinda* reset on movement of the cup/piston.

It's the master cylinder. I can give you probably 1100 anecdotal experiences where brake lever/pedal went down and in 100% of the cases where a leak wasn't present, it was the master cylinder. Proportioning valves on cars don't do this. ABS systems on cars and bikes don't do this. Leaking things don't do this.

It's the master cylinder.

I turned a wrench from '81 to '91 professionally, and as a side gig since then on everything with wheels and an engine.

It's the master cylinder.

Don't get stuck in analysis paralysis. Just replace it.

PS... it's the master cylinder.
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Offline Max Wedge

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Re: Intermittent Front Brake Failure
« Reply #18 on: May 16, 2019, 07:30:49 PM »
It takes time to bleed down thru the shaky seal.After you two pump it t stays pumped up for a while and slowly bleeds past the seal till you have nothing at the initial lever pull.Same as car brakes ,( pre abs )  you could pump the pedal up by repeated stepping on it.

I'm with you, but wouldn't it be somewhat consistent? It can sit for days in the garage, no problem, but 10 minutes on the road, and then not again until 50 minutes on the road?  I can see vibrations causing it, but i would think that it would be every ten minutes or every 20. And not doing it at all when parked?

Just thinking, because i'm work and its at home.

Any sure way to test it before I tear it apart?

It's the master cylinder. Any... EVERY... other option would produce a leak. The master cylinder's push cup is worn, hardened, maybe even got a slight crack in it. Double-pumping it pushes enough fluid to give you some resistance; it's just a flow issue. you're trying to push more on the second pump than will push through/past any gaps that have *kinda* reset on movement of the cup/piston.

It's the master cylinder. I can give you probably 1100 anecdotal experiences where brake lever/pedal went down and in 100% of the cases where a leak wasn't present, it was the master cylinder. Proportioning valves on cars don't do this. ABS systems on cars and bikes don't do this. Leaking things don't do this.

It's the master cylinder.

I turned a wrench from '81 to '91 professionally, and as a side gig since then on everything with wheels and an engine.

It's the master cylinder.

Don't get stuck in analysis paralysis. Just replace it.

PS... it's the master cylinder.

So...not being able to leave well enough alone, and having a tequila or two, I went out to the garage to poke at it. I rode it (ha, no, I know better than that) I lifted the front tire and backed off the brake pads (yes still lots of meat on them) and tried to see if disc were floating properly. I saw nothing wrong but while pumping them back, it failed. Huh. Not running, or rolling down the road. I thought it might have just been my imagination, so I tried it a couple of more times, and was able to recreate it. If I didn't let the lever all the way back, I could get it to stay in failed mode. Releasing it all the way, would allow it to bite. I can't see it being anything other that the .....master cylinder. I am now 95% certain.
 
  So now the discussion moves to: can I get a rebuild kit for it? Best place to get one from? How hard are they to rebuild? Any part that wear that would require honing/sleeving? It's been a long time since I rebuilt one, and that was for a car. A new one is $479! I have a wedding in two weeks with a rehearsal  dinner being hosted here at the "Two-Stroke Pub" (my basement entertainment venue) so we are in the middle of landscaping/cleaning/painting AND I really need my two wheeled therapy at this time, and I have very little to spare, much less having parts all over the garage. I will spend a little for the convenience, but I'm kind of choking on $500 for parts alone.
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Re: Intermittent Front Brake Failure
« Reply #19 on: May 16, 2019, 07:52:02 PM »
They're easy to rebuild. Usually it's a spring/ring clip, a piston, a seal and a cup, with a dust cover. Doing the work isn't the big deal. Doing the work while clean and knowing what to look for is the bigger issue.


9 times out of 10, maybe 8 times out of 10, it's a temporary solution. The seals go bad because of rust or dirt in the cylinder, and there is scoring in the bore that will rear its ugly head again at some point.

"The only variable is time." -- Quentin Barnes, GFL Series, Scott Sigler

For a life-critical thing like brakes, certainly for the master cylinder, I'd replace new. I've rebuilt plenty of calipers with great success, and some master cylinders, too, but I completely believe it's a band aid.

If you want to try a rebuild, go to a decent auto parts store, and get a proper parts cleaner dunk tank (usually sold in 1-gallon cans with a basket, around $35), and some soft bristle bore brushes.

Or just order this: https://www.amazon.com/Berryman-Products-0996-Arm-Chemical-Fluid_Ounces/dp/B077Y1YSZ5

You want to make sure the bore is pristine before you even think about putting it back together. Then get some brake cleaner to clean up after you water-rinse the dunk cleaner off, then immediately coat the inner bore with brake fluid (clean, new) and inspect it as carefully as you can. If you see ANY scoring along the length of the bore, just buy a new one. It's just not worth futzing with; you don't have and don't want to pay for the stuff to re-hone it. Plus, it's aluminum, and require a finesse that most machine shops used to doing cars won't or can't do.

I can't really over emphasize how critical it is to just skip the rebuild if there's any scoring.

If not, and you just have worn rubbers, go for it.

But!!!!!!!

FULL BLEED!

All DOT-class brake fluids EXCEPT 5.1 are alcohol-based and their job is to capture water; bleeding the entire system is critical. Get all the water out and gone. FULL bleed.

If you don't wanna tackle it, send it here; I'll hand it off to my buddy to do. He owes me a favor or two. :)
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Re: Intermittent Front Brake Failure
« Reply #20 on: May 17, 2019, 01:41:03 AM »
Bubbles is right. But he left out that Chinese aftermarket replacements are silly cheap and work just fine.
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Re: Intermittent Front Brake Failure
« Reply #21 on: May 17, 2019, 07:34:37 AM »
But he left out that Chinese aftermarket replacements are silly cheap and work just fine.

True,  and I've done it. Plenty. But I never feel good suggesting it. You just never know. :\
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Offline Max Wedge

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Re: Intermittent Front Brake Failure
« Reply #22 on: May 17, 2019, 12:37:46 PM »
I work for a Chinese car company. Devil>< details.
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Re: Intermittent Front Brake Failure
« Reply #23 on: May 18, 2019, 07:35:21 AM »
I would bite the bullet and replace,as bubbles noted,rebuilding is at best a bandaid,and unless you hone the bore perfect,and don't increase the diameter much ,it still doesn't last in many case's.
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Re: Intermittent Front Brake Failure
« Reply #24 on: May 18, 2019, 11:39:53 AM »
Check with Beemer Boneyard and see if they have a good takeoff?


https://www.beemerboneyard.com/32727727031.html


Never mind, they're sold out.
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Offline Max Wedge

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Re: Intermittent Front Brake Failure
« Reply #25 on: Yesterday at 02:05:08 PM »
Check with Beemer Boneyard and see if they have a good takeoff?


https://www.beemerboneyard.com/32727727031.html


Never mind, they're sold out.

That says something.
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Re: Intermittent Front Brake Failure
« Reply #26 on: Yesterday at 02:08:15 PM »
« Last Edit: Yesterday at 02:24:53 PM by sleazy rider »
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