Juicy 7 Rotor rubbing on calliper casing.

Dave F.
Dave F. Posts: 41
edited July 2008 in MTB workshop & tech
Hi

I've got Juicy 7 on a Stumpy FSR.

http://i279.photobucket.com/albums/kk129/DaveF63/Bike/PICT0006.jpg
(Sorry, it's a bit blurred)

If the image loads correctly you see that the top pad appears to have worn down unevenly.
So much so that the rotor is rubbing on the calliper casing. This is after i tried realigning the calliper as per the instruction manual.

What's causing this?
Should the piston at the bottom stick out that far?
I've tried pushing it back in with a plastic tyre lever but it won't budge.

Also, when I have the bike upside down, & with the brake calliper attached, I loose pressure when I squeeze the lever.
As it's should be full of brake fluid with no air (I've bled it a few times) I'm surprised this is happening.
Any ideas why?

Is there a way to upload images directly to the website?

Hope you can help.

Dave F.

Comments

  • Matt 24k
    Matt 24k Posts: 186
    It looks like one of the pistons is sticking. The brake fluid will always take the path of least resistance and push the piston and therefore pad which moves the easiest.
    The first thing to note is that if you are not confident or competent you should not mess with your brakes! There is an excellent manual from Avid, http://www.sram.com/_media/techdocs/200 ... 00RevA.pdf However, if you want to try some basic stuff then the following may be of use.

    1) Get a bowl of warm soapy water and an old toothbrush. Submerge the whole caliper with the pads removed and gently brush away any dirt.
    2) Very carefully pump the brake lever to push the brake pistons outwards. You do not want the pistons to come all the out, so take it easy. You will probably find that one piston moves much more than the other.
    3) Try holding the free moving piston into the caliper with something either wooden or plastic so as not to damage the piston.
    4) Repeat step 2 but really slowly this time as you may find that your sticky piston suddenly unsticks itself and comes flying out along with all the fluid!
    5) Using your toothbrush and soapy water clean all around the freshly exposed part of the piston but try to wash debris away from the seal rather than into it. The longer you soak the exposed part of the piston the better.
    6) Rinse the caliper and apply a very light coating of silicone grease to the exposed piston outer walls. This is a tip from my LBS and it seems to work.
    7) Push the pistons back and then check to see if they move more freely. You will then need to replace the pads and realign the caliper.

    If you are not sure about any of the above then take your bike to your LBS and they wiill do a far better and safer job. They will probably just do a full overhaul and replace the seals and pistons.
    HTH
  • Dave F.
    Dave F. Posts: 41
    Thanks for the reply Matt

    Forgot to say it's an '07 stumpy.

    I had downloaded last years equivalent of your manual, but decided to post here first, before embarking on a complete daunting dismantle.

    I shall give your suggestions a go now.

    As you can see from the before/afters photos, I believe you're correct. The top piston does budge, but also the bottom one isn't returning to its seat. i have to really push it hard with a screwdriver. Would this be due to a ddgy spring?

    Before:
    http://i279.photobucket.com/albums/kk12 ... CT0011.jpg

    After:
    http://i279.photobucket.com/albums/kk129/DaveF63/Bike/PICT0012.jpg

    I'm still confused as to why i loose pressure in the lever when i turn the bike upside down. it takes a few minutes of furious pumping, when righted, to get any firmness back.

    Any recommendations for replacement pads?

    Cheers
    Dave F.
  • Matt 24k
    Matt 24k Posts: 186
    First up the brake system is hydraulic. Basically, you squeeze the lever which puts pressure on the brake fluid which pushes on the piston behind the pad. There are no spings behind the piston. The fluid is in a vacuum and that sucks the piston back in when you release the pressure on the lever.
    You say that you lose pressure when you turn the bike upside down so I would imagine that you have air in your system which is not good. It needs bleeding which really requires a kit from Avid or a semi bodge!
    Make sure that your master cylinder cover is facing upwards so that the fluid does not drain out. Then after removing the master cylinder cover push the pistons back squarely and evenly. This will allow most of the air to expelled upwards. Fit the new pads and put the caliper on the disc and cable tie the lever back to the handle bar. Also try tapping the hose around any fittings. Do not leave the cover off for long as it is really easy to contaminate the fluid. Then refit the master cylinder cover and leave the cable tie on over night.
    The above does not constitute a proper bleed, that is shown in the Avid manual, but it usually improves the feel at the lever.
    As for pads, it's all down to personal preference. I tend to find the original pads work the best and they are designed to be kinder to the disc etc. You then get into the whole sintered, organic or whatever thing which all comes down to use and personal preference.
  • Dave F.
    Dave F. Posts: 41
    Matt 24k wrote:
    First up the brake system is hydraulic. Basically, you squeeze the lever which puts pressure on the brake fluid which pushes on the piston behind the pad. There are no spings behind the piston. The fluid is in a vacuum and that sucks the piston back in when you release the pressure on the lever.
    I meant the spring pad clip. as Avid call them in their manual, but obviously they're just to hold the pads against the piston.
    You say that you lose pressure when you turn the bike upside down so I would imagine that you have air in your system which is not good. It needs bleeding which really requires a kit from Avid or a semi bodge!
    I should of said I've already bled them using the syringe kit 4 times & still have the problem! When the pressure returns they are quite stiff, not spongy as I'd expect with air still in them, although I do have large amounts lever travel before they bite.
    Make sure that your master cylinder cover is facing upwards so that the fluid does not drain out. Then after removing the master cylinder cover push the pistons back squarely and evenly. This will allow most of the air to expelled upwards. Fit the new pads and put the caliper on the disc and cable tie the lever back to the handle bar. Also try tapping the hose around any fittings. Do not leave the cover off for long as it is really easy to contaminate the fluid. Then refit the master cylinder cover and leave the cable tie on over night.
    The above does not constitute a proper bleed, that is shown in the Avid manual, but it usually improves the feel at the lever.
    I'm a bit confused. Do you mean the cover to the reservoir as shown on page 10 Figure 6 of your manual or the bleed port screw?

    As far as i can see I have 3 problems:
    1. One piston is stuck.
    2. The other piston won't return (lack of vacuum?)
    3. Possible air the the system.
    As for pads, it's all down to personal preference. I tend to find the original pads work the best and they are designed to be kinder to the disc etc. You then get into the whole sintered, organic or whatever thing which all comes down to use and personal preference.

    Thanks for your time
    Dave F.
  • Larok
    Larok Posts: 577
    edited July 2008
    OK so your calipers need sorting and new pads

    http://www.wiggle.co.uk/p/Cycle/7/Avid_ ... 300003709/

    You either need to pay some one or learn how to both fix that stuck piston. It has gotten stuck against its housing and needs to be freed up and returned to normal if possible or perhaps replaced.

    I think you need to sort the piston out before you can bleed it properly anyway.
    My Ride Stumpjumper Expert 2008
  • Matt 24k
    Matt 24k Posts: 186
    Sorry I didn't read your original post properly or I would have realised that you have a bleed kit and have used it. So you can disregard the bodgers guide to semi bleeding apart from cable tieing the lever to the bars over night to allow any air to rise to the master cylinder. I am away from the bike that has Avid brakes on at the moment so I cannot check where the master cylinder cap is. However, just cable tieing with it in place will help if you do not have a leak.
    If after bleeding the brakes air is getting back into the system then you must have a leak some where. Clean everything thoroughly then with the brake system fully assembled cable tie the lever to the handle bar and leave it over night. Then check for any signs of escaped fluid around hoses, joints, pistons, master cylinder, bleed ports every where.
    If you can't unsieze the piston then you will be into a full strip down and replacement which is covered in the manual or maybe 25 quids worth of labour at your LBS.

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