Front Brakes: Linear=Good StepFunction=Bad

MisplacedTexan · March 02, 2006, 06:10:14 PM

I'm riding an 04 GS with 3500 miles on it. My front brake seems to have developed a problem- whenever I pull on the lever, it's linear up to a point, and then I have to pull significantly harder to get the lever to "break free" and gain more braking power. It's as if the lever has about four finite braking positions, but nothing in between. It makes it difficult to stop smoothly and feels as if the brakes are sort of sticking. The problem has to be in the master cylinder, but I don't know what's causing it. The problem has gone away and come back twice. I don't see how anything in the master cylinder could be worn with only 3500mi on the bike. Oh, and the brake fluid was changed 6 months ago and I keep the bike outside. Any thoughts?

RVertigo · March 02, 2006, 06:45:08 PM

I have this same problem... But, if I put down pressure when I pull the brake, it's smooth again.

I've been meaning to try to do something about it... I was thinking about lub'ing all the parts to see if it fixes it...

But, I'd love a better idea.

Egaeus · March 02, 2006, 06:46:16 PM

It could also be the caliper piston binding. Perhaps it is corroded? How may pistons does the GS have? I haven't done any brake work yet. If it's more than one, then it probably would be in the master cylinder.

RVertigo · March 02, 2006, 06:49:08 PM

The problem I have is in the actuall brake lever... Same for you MisplacedTexan?

MisplacedTexan · March 02, 2006, 07:08:42 PM

Quote from: RVertigo on March 02, 2006, 06:49:08 PM
The problem I have is in the actuall brake lever... Same for you MisplacedTexan?

I don't think so, but I can't say for sure. To me, it feels like the piston in the master cylinder is binding. Like Egaeus said though, I guess it could be the caliper piston binding, and I'm just feeling that feed back through the fluid, mc piston, and lever. Is there any part of the brake system that needs lubrication that I'm forgetting?

starwalt · March 02, 2006, 07:10:42 PM

Quote from: Egaeus on March 02, 2006, 06:46:16 PM
It could also be the caliper piston binding. Perhaps it is corroded? How may pistons does the GS have? I haven't done any brake work yet. If it's more than one, then it probably would be in the master cylinder.

+1 to this. I just rebuilt the front master cyl, but haven't filled with fluid yet. The day of the build, I coated the new piston with a little brake fluid and everything went smooth. I put it on the bike and went to bed.

The next night I decided to check my handywork. The silky smooth pull was replaced by a hurky-jerk! I pulled it apart again and found one of the rubber rings had rolled over and was binding.

Have you replaced the brake fluid on this bike? Assume no one ever has. Kerry's page has a brake fluid replacement tutorial and I know for sure it is here or off the main GS page.

QuoteIs there any part of the brake system that needs lubrication that I'm forgetting?

Only minor brake grease on the sliding portion of support arms at the caliper. Otherwise the brake fluid itself, a specialized oil, is the lubricant for the master cylinder and the pistons.

The DOT brands recommended are hygroscopic and need to be changed almost annually.

MisplacedTexan · March 02, 2006, 07:47:44 PM

How many rubber rings are on the piston? I searched for a rebuild thread on here and looked on Kerry's site and the main site, but couldn't find any pictures or rebuild instructions. The bikebandit oem schematic has the cup,piston,springs, and I guess rings, listed as one part ($17.19 by the way). I think I'm gonna just take the master cylinder apart this weekend.

aqxea2500 · March 02, 2006, 07:51:52 PM

I had the same problem all I did was loosen the screw in to of the lever and used a q-tip to clean the crap put and it works great now, make sure to tighten the screw after though.

Egaeus · March 02, 2006, 11:28:29 PM

By the way, if you're going to describe your brake behavior as a step function, you should at least give us plots of the step response and the impulse response. How are we supposed to analyze the system without that?!

Hope you get it fixed, and you probably shouldn't ride until you do. Unintentional stoppies can be hazardous to your health.

scratch · March 03, 2006, 10:41:18 AM

Minor details to check, but yeah grease the pivot bolt. If the bike has been down, and the lever is loose vertically, or you can wiggle it up and down, it might rub up against the inside of the m/c housing as it pushes in the m/c. I dab a little grease on that end as well.

RVertigo · March 03, 2006, 11:22:27 AM

Quote from: scratch on March 03, 2006, 10:41:18 AMMinor details to check, but yeah grease the pivot bolt. If the bike has been down, and the lever is loose vertically, or you can wiggle it up and down, it might rub up against the inside of the m/c housing as it pushes in the m/c. I dab a little grease on that end as well.

<-- I think this is my issue.

sledge · March 03, 2006, 01:30:59 PM

Once saw a similar thing on another bike I owned. The hole in the lever that the pivot pin sits had worn oval and so had the pin, as a result the lever would move so far, then stick, then free itself with a jolt. OK 3.5k isnt a lot of miles but its worth looking at.

MisplacedTexan · March 03, 2006, 05:46:20 PM

Problem fixed!! I took the pivot bolt out, removed the brake lever, and generously applied grease to all surfaces. I also dabbed a bit of grease on the outer surface of the m/c piston where the lever pushes it. After that, the lever motion was smooth as butter. Thanks to everyone for helping me out.

annguyen1981 · March 03, 2006, 09:24:40 PM

I've had my '04 since last fall. I bought it with 1500 miles, and I've put 2500 miles since then... (for those with bad math skills, that's 4000k miles)

I've had no problems with brakes yet.

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Front Brakes: Linear=Good StepFunction=Bad