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  1. #1
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    Where are the Dust Sheilds?

    Do any of the current crop of calipers use dust shields around their pistons to protect their seals?

    All automotive and motorcycle calipers use dust shields in front of their piston seals because the brake dusts and road grime will break a seal down in a few months, which will result in a frozen piston and brake drag.

    My Avid J7 caliper seals last for about 3 months before they need to be replaced or cleaned. Rebuilding a J7 caliper every few months is time consuming and expensive (rebuild kits retail for around $40.00 per caliper). If the J7 caliper utilized a dust shield around the piston, then I believe the pistons would not have any sticking issues for many years.

    Solutions:

    1) One temporary solution would be to glue a thin heat resistant rubber cover over each piston, which would keep the grime out of the seals for a wile until the glue came undone.

    2) A better solution would be to buy a disc brake with a dust shield.

    3) Use disc brakes with cheaper rebuild kits like Hayes.

    4) Forego the hydraulic brakes until they are more refined in a few years and go with mechanical disc brakes.

  2. #2
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    Are you talking about the rubber boot around the piston on a car/moto's pistons? I too thought about that on a bike's caliper.

  3. #3
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    It's £4 for a Hope caliper overhaul kit, although I've never known anyone need to replace seals after three months. Three years, perhaps.

    Your problem isn't universal, or even widespread, so I doubt that you'll ever see a 'solution'. If your riding conditions and/or braking style mean that you're getting/generating a lot of dust, I'd suggest either buying better brakes and/or getting into a maintenance routine. Having to clean a caliper interior every three months isn't, I don't think, that big of a deal. You could try using sintered pads if your current ones are producing that much dust.

    Out of interest, what did your shop say about their advisory to use mineral oil to clean/lube your pistons/seals?
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  4. #4
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    Yes, I am talking about the rubber boot around the piston. I was talking to an old-timer mechanic who has owned and worked on lots of older cars and motorcycles in his life time about my sticky piston problem with bicycle brakes. He said that the first generation of automotive and motorcycle disc brakes all had the same problems that I am having with my bicycle disc brakes. Most of the early disc brakes systems malfunctioned after a short time and needed to be rebuilt or replaced within 10K miles. It took the industry about five years to refine their disc brake systems, which included enhancements such as better rubber compounds, better piston seal groove profiles and adding dust shields over the piston seals. These enhancements along with many others have resulted in a very durable system that can last for hundreds of thousands of miles before servicing.

    Why is the bicycling industry so slow to make these same changes? Possibly because, most bicycles are not ridden that often and therefore do not need to be built for durability.

  5. #5
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    SteveUK, the bike mechanic that recommended mineral oil to loosen up my Avid J5 and J5 pistons, maintains that it was under advisement from Avid. The use of mineral oil does contradict the information in Avidís 2008 Technical Manual, which says adding or filling their hydraulic circuits with mineral oil or DOT 5 silicone based brake fluid will ruin the internal seals and cause the pistonís to stick. Avidís seals are compatible with DOT 4 and 5.1 brake fluids only. The section on sticky pistons in Avidís manual, recommends pushing the piston back in its cylinder a few time to free it up. If this procedure does not work than the caliper will need to be rebuilt. Avid does not directly recommend using brake fluid or any other type of lubricating medium to unlock their pistons.

    Brake pad drag, uneven lining wear, lack of power, pulsation, NVH or vibration, lack of modulation or on-off power, pads-to-rotor alignment issues and slow returning levers are all symptoms of sticky caliper pistons. There are a lot of riders in this forum, which have had these issues listed above but never traced the problem to its root cause. One such posting is the Avid Juicy Turkey Warbler Thread.

    I stumble across the dust boot idea when I read an automotive brake bulletin that linked a frozen piston, which had a contaminated seal, with a torn dust shield.

  6. #6
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    Quote Originally Posted by SteveUK
    It's £4 for a Hope caliper overhaul kit, although I've never known anyone need to replace seals after three months. Three years, perhaps.

    Your problem isn't universal, or even widespread, so I doubt that you'll ever see a 'solution'. If your riding conditions and/or braking style mean that you're getting/generating a lot of dust, I'd suggest either buying better brakes and/or getting into a maintenance routine. Having to clean a caliper interior every three months isn't, I don't think, that big of a deal. You could try using sintered pads if your current ones are producing that much dust.

    Out of interest, what did your shop say about their advisory to use mineral oil to clean/lube your pistons/seals?
    I think dust from dirt will be more of a problem than brake dust. I've had to free stuck pistons on my 4pot XTs about 3 times now.

    Maybe one reason why there is no dust boot on the piston is due to it being so small.

  7. #7
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    You could try silicone grease (Plumber's Grease) on the piston to lube the seals. Silicone grease is compatible with pretty much any rubber, except silicone rubber. Pull the lever until the pistons move out a little bit and put some grease on it. Push it back in and wipe all the excess grease off.

    I have Juicy 5s on both my bikes with no problems after 2+ years. I ride at least 20 miles a week off-road. Do you have unusual soil or do you ride in very harsh conditions?

  8. #8
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    Yes I tried automotive silicon brake prep grease which is compatible with DOT 3 and 4 seals and silicone oil in a spray can on the edges of the extended pistons. Unfortunately the silicone did not free the sticky pistons. Dirt is building up in the front side of the seal groove and working its way down to the bottom of the seal. The sealís movement against the front of the bull-nosed grove gets more and more restricted as more and more dirt builds up in this area. When a portion of the seal cannot flex foreword with a lever stroke it create a wedging force against the side of the piston. Pouring rubbing alcohol, brake fluid or silicon oil on the sides of the pistons and in the front of the seal groove does not flush out the dirt it just moves the dirt deeper in the groove. Having rebuilt two Avid Juicy calipers so far, I can tell you that it only take a small amount of dirt to gum up the works. The heat combine with dirt and any brake fluid residue creates a small thin spot of glazing compound that must be scrapped out of the groove before a new seal is installed. Like I said in the previous post, I get about 2 to 3 months of service out of the brake before the pistons start freezing up.

    The next time I rebuild an Avid caliper, I am going to try packing the front of the seal with silicone brake-prep grease to provide a barrier against the dirt. This grease barrier should work every time the piston pushes out; however the contaminated grease will work its way into the seal groove every time the piston contracts back into its bore. Anyway, I am hopeful that it will last longer than 3 months. A dust boot would resolve this dirt build-up issue.

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