Topic: Constant brake issue

[Finley]

Hello all, I have been plagued with a pestering front brake issue. It has rubbed since I rebuilt the bike a little over a year ago. I have since rebuilt the brake caliper twice and replaced the brake lines with SS braided ones, after a few weeks it starts rubbing to the point it feels like my brake is being engaged. I don't know what the problem is. Any ideas? Any help is appreciated.


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[flybox1]

During your rebuild, did you replace the piston o-ring and clean out its groove?
Have you cleaned your master cylinder and the fluid return holes in its cup?

These two issues account for the majority of caliper malfunction

Your issue is with the piston not retracting.  Things that can lead to this:
pitted piston
bad/old/piston o-ring, crudded up groove
master cylinder fluid return hole clogged. 
improper caliper arm to rotor gap.  remove the caliper halves and check for caliper arm adjustment range.  the spring should hold it in place, but allow pressure to 'swing' it.  check/set your pad to rotor gap.

[DH]

And make sure the front axle is installed from right to left, with
hex/spacer nut installed on the left side, hex portion against inner
fork leg. It affects wheel offset. Anyone have a pic of this maybe?

[Duanob]

You say you rebuilt the caliper twice but have you done anything with the master cylinder? As Flybox1 says the little fluid return hole can get plugged and the pressure with remain in the lines. You might need to rebuild the MC, use a Honda kit.

[Finley]

I have replace to seal and o ring in the caliper and cleaned the groove out. I haven't rebuild the MC, I used a wire and tried making sure the holes were clear. I'll double check the axle as well. That could explain a few things that have been going on. Would the wheel spacer being in the wrong place make the front end feel like is has flex?


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[DH]

I have replace to seal and o ring in the caliper and cleaned the groove out. I haven't rebuild the MC, I used a wire and tried making sure the holes were clear. I'll double check the axle as well. That could explain a few things that have been going on. Would the wheel spacer being in the wrong place make the front end feel like is has flex?


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The nut being installed backwards was just another suggestion you might want to verify. It'll affect caliper to rotor alignment. Just one possibility, among the others that the guys have already suggested. And this answer is intended for 750k and 500/early 550k. Not sure about others. . Might want to do a search for front end flex. Lot of other things could cause that.

[Finley]

Alright guys.... I took the advice posted here and rebuilt the MC, I also cleaned/rebuilt the caliper and flushed the lines and cleaned and lubed the brake arm. I bled the brake up and no success, The piston still isn't releasing from the rotor and the brake arm spring doesn't seem to help push the brake back to center. Would it be wise to replace the MC? Or the caliper? Both? I'm out of ideas that could be the problem. Any help is appreciated.

Thanks.

[goldarrow]

Did you replace the seal inside the caliper bore?

[Henning]

Does the piston-side brake pad move freely inside the caliper? Some have had problems with aftermarket pads that are too big and you have to sand off some paint to get them to move freely in the housing. Plus you need to apply some brake grease between the pad and the caliper housing.

[Finley]

I replaced the seal inside the caliper about 6 months ago. The piston does push in smooth with little resistance, I have already sanded the paint off the pads. I found out that the paint gums up in there and seizes the hard way. Haha.


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[mrbreeze]

You really haven't said how you cleaned the return hole in the master cylinder reservoir. There are 2 holes . One is very small. That is usually the one that gets plugged. Get a # 1 guitar string and run it through that tiny hole. That's what fixed mine many years ago.

[Finley]

I cleaned the reservoir holes with a guitar string. When the cap is off and I squeeze the lever I can see the fluid move from the return hole.


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[strynboen]

have you mounted the front fender on the right side of caliper mounting..it can give a  sidedrag if it are on the vrong side..the caliber mount vill be moved a fev milimeters and not line korret up to the disc

the caliber piston move just 0,1 mm back vhen relese the breake..it can  hardly not be seen..test it turn the hveel by hand..must run absolut free

[Finley]

I'm pretty sure the fender is mounted right, the brake arm is against the fork lower and the fender is mounted to the inside of the brake arm. When setting up the brake arm, should the piston pad be set up closest to the rotor or should the dead pad be set closer?


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[Bodi]

The arm adjustment is supposed to locate the dead pad just off the disc, 1/4-1/2 turn back from where there's drag. This is a bit tricky with the locking nut as tightening it changes the adjustment - but it's not impossible.
The two things that usually cause this problem of piston non-retraction are corrosion in the seal groove and air in the system.
Corrosion causes increased friction at the seal. Air frustrates the pull-back suction from the master cylinder.
When you pull the brake lever, the first fraction of movement just blows fluid back to the reservoir through the relief hole. Once the hole is covered by the piston you are pumping fluid into the brake line, pressurizing the system. If the caliper is working properly and there are no air bubbles, the caliper piston slides freely out until the disc is getting squeezed - after that the brake applies proportional to the pressure.
Then you release the lever.
The seal distorts slightly under pressure. When the pressure is released the seal relaxes, pulling the piston back a hair. Once the m/c piston has released pressure it moves back to where the relief hole is uncovered again. In that movement it produces a suction. This suction moves the caliper piston back to where it was before brake application, off the disc. The arm spring relocates the caliper so the dead pad is clear.
With air in the lines, on brake application it gets compressed and the bubbles shrink until the pressure is high enough to move the caliper piston. Actual braking power is unaffected, lever travel increases and if extreme the brakes will e barely on with the lever against the bar.
The problem is that the compressed air bubbles expand again as the lever is releases, meaning the fluid is under pressure from them rather than under suction when the caliper piston is returning to the relief hole. That means the caliper pad does not retract. The brake never releases fully until riding vibration rattles the caliper around enough to bounce it free. There's no real resistance to m/c piston retraction, fluid can go through the relief hole. A corroded seal groove makes it harder to move, though.
If the relief hole is truly plugged, the situation is similar but can get very serious - the brake is left slightly dragging and heats up, the heat expands the fluid, the brake gets tighter and hotter, and eventually you have heavy braking applied and a red hot disc if you don't notice it happening and stop. Hitting a slippery bit of road with the brake dragging severely gets quite nasty.

[calj737]

Finley - just re-ready our post. I'm just confirming a point here; you do not indicate that the piston itself is either new, or in perfect condition. You've replace dot he O-ring, great. The MC is rebuilt, new lines, etc. But the piston cannot tolerate pits, rust or deterioration either.

So what is the condition of the piston? If questionable, replace it with a new unit. Have no doubts about it being perfect; it must be perfect else you will risk injury and suffer ongoing issues. If you on need a new one, many members use the Phenolic pistons sold by a member here.

If your piston is fine, then somewhere you've got a flow restriction. Only explanation for dragging and sticking. To that end, have you attempted to bypass the front T switch to see if that unit is compromised?