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Nissin front brake master cylinder free play

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Hi tt. So i noticed the other day that the front master cylinder on my yz250f has travels about half way through its stroke before it even begins to to push on anything. I can move the lever a solid inch in with absolutely NO grab. The line is totally bled cause when it gets to the end of that “free play” its rock solid. What can i do to fix that?

thanks!

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I have found that a brake caliper rebuild with new seals can help with this sympton

It is the seals which pull the pistons back into the caliper body when the lever is released.

Instead of the pistons sliding past the seals when the lever is pulled and the seals only distorting slightly resulting in minimal piston retraction, the seals seem to grab the pistons more  resulting in a greater degree of distortion during braking before they slide resulting in more retraction when the lever is released

 

I think its called stiction, and new seals reduce the level of this

Edited by GuyGraham
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6 hours ago, Will05W. said:

Ohhh, those are what allow the caliper to “float” back and forth right?

Yes

4 hours ago, GuyGraham said:

I have found that a brake caliper rebuild with new seals can help with this sympton

It is the seals which pull the pistons back into the caliper body when the lever is released.

Instead of the pistons sliding past the seals when the lever is pulled and the seals only distorting slightly resulting in minimal piston retraction, the seals seem to grab the pistons more  resulting in a greater degree of distortion during braking before they slide resulting in more retraction when the lever is released

 

I think its called stiction, and new seals reduce the level of this

Yup, dirty pistons, old seals can do this as well, but to a lesser degree. The seals act like a ratchet, somewhat. Flexing until the wear of the pads is so much that the seal cannot flex anymore and the piston moves a tiny bit against the seal, the self adjustment. Usually enough for the pads to barely make contact with the rotor. The difference between the seal flexing and then the movement is extremely small and not apparent at all. Though a stuck seal will make the distance span between flexing and self adjustments longer, it still would be relatively insignificant.

If this were my bike, I'd put in new, properly greased sliders, ensure they move smooth and free and rebuild the caliper. Note, when rebuilding it, after it is all clean., be sure to 'wet' all the parts, caliper bore, seal and piston with brake fluid. Pour a little fluid in the bore. Gently work the piston in (cover the port where the fluid line attaches). As soon as the piston is barely inside the seal, pour a little brake fluid in the port. Not a lot, just a few CC's. Hold the cailper so the port is the highest point. Tap with a handle of a screwdriver to free any air bubbles. Gently press the piston all the way in. Then install the caliper, regreased pins and brake line. New pads. Bleed.

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6 minutes ago, William1 said:

 when rebuilding it, after it is all clean., be sure to 'wet' all the parts, caliper bore, seal and piston with brake fluid. Pour a little fluid in the bore.

I found using brake fluid to wet the seals, results in them attracting moisture (as brake fluid is hygroscopic) and then that horrible white crud builds up esp behind the seal forcing it outwards and putting more pressure the piston and increasing the stiction. Eventually the piston is all but seized.

I now use red rubber grease to lube the seals and have found this eliminates the white crusty deposits that build up behind the seal.

Most certainly use red rubber grease on the rubber boots over the sliders as normal lithium grease seems to make them swell up and stopping the floating action

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2 minutes ago, GuyGraham said:

I found using brake fluid to wet the seals, results in them attracting moisture (as brake fluid is hygroscopic) and then that horrible white crud builds up esp behind the seal forcing it outwards and putting more pressure the piston and increasing the stiction. Eventually the piston is all but seized.

I now use red rubber grease to lube the seals and have found this eliminates the white crusty deposits that build up behind the seal.

Most certainly use red rubber grease on the rubber boots over the sliders as normal lithium grease seems to make them swell up and stopping the floating action

When possible, I use silicone fluid that is non-hydroscopic as well. Also, once the caliper is together and just before I put the pads in, I spray it down with brake Kleen so no fluid is on the exterior.

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Hi all, thanks for the replies. I wanted to update this thread cause i finally got the time to look at those brakes. So heres what i found. No crud or corrosion on the slider pins, they move freely. Took the pads off at operated the lever. The pistons do in fact move in that “free play” area so its obvious that the pistons are being pulled too far back into the caliper when the lever is released. Is replacing the pads a good place to start? They’ve never been replaced but theres still 4-6mm of material on there. Manual doesn't have a minimum thickness

thanks!

also, oem pads or whats a good aftermarket?

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46 minutes ago, Will05W. said:

Hi all, thanks for the replies. I wanted to update this thread cause i finally got the time to look at those brakes. So heres what i found. No crud or corrosion on the slider pins, they move freely. Took the pads off at operated the lever. The pistons do in fact move in that “free play” area so its obvious that the pistons are being pulled too far back into the caliper when the lever is released. Is replacing the pads a good place to start? They’ve never been replaced but theres still 4-6mm of material on there. Manual doesn't have a minimum thickness

thanks!

also, oem pads or whats a good aftermarket?

Seals not pads need changing

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pads are cheap.  just get new ones while you are there. also check the pin that holds the pads in.  if that is grooved it can cause issues. 

ALso for the slide pins, you can use the automotive specific caliper grease for those.  Works well. 

Edited by JJRace
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Hey guys. Just got finished replacing the piston seals and the free play is still there, and a little worse than it was before (but some of that is a little residual air).

any other ideas?

Also, do the seals have an orientation? The manual didnt say any thing about it and they looked pretty symmetrical so i just threw them in. 

As always, thanks for the help!

just as a side note cause ya’ll might get a chuckle out of it, i shot myself in the face with a piston cause i didn't think to put a finger over them when working them out with compressed air :doh:

Edited by Will05W.
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Strange, new seals should have improved it

 

There is an orientation to the seals, but for some reason its not in the WSM

The caliper is the same as used on the DRZ400 (everything interchanges - I use Yam seals in my DRZ caliper as they are cheaper than buying from Suzuki)

In the DRZ manual it shows you this

Capturea.JPG.d0088f1760b459403ce44013fbe18471.JPG

Worth checking and seeing if it improves the excessive free play you still have.

I use the camera on my phone, zoomed in, to identify which way way round the taper is as its quite difficult to see by eye

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8 hours ago, GuyGraham said:

Strange, new seals should have improved it

 

There is an orientation to the seals, but for some reason its not in the WSM

The caliper is the same as used on the DRZ400 (everything interchanges - I use Yam seals in my DRZ caliper as they are cheaper than buying from Suzuki)

In the DRZ manual it shows you this

Capturea.JPG.d0088f1760b459403ce44013fbe18471.JPG

Worth checking and seeing if it improves the excessive free play you still have.

I use the camera on my phone, zoomed in, to identify which way way round the taper is as its quite difficult to see by eye

so the angled portion of the seal slants down away from the pads and faces away from the pistons?

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2 hours ago, Will05W. said:

so the angled portion of the seal slants down away from the pads and faces away from the pistons?

Yes

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Ok thanks.

also, my caliper bracket (The piece that bolts to the fork and has the slider pins) is scraping on my rotor which it never did before

i dont know what to do to fix that. I just rebuilt my forks and had a new front tire put on the front wheel.
 

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14 hours ago, Will05W. said:

Ok thanks.

also, my caliper bracket (The piece that bolts to the fork and has the slider pins) is scraping on my rotor which it never did before

i dont know what to do to fix that. I just rebuilt my forks and had a new front tire put on the front wheel.
 

The wheel spacers are different - you may them in the wrong sides, causing the wheel to be offset

Edited by GuyGraham
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