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Obvious disk brake question...

by sergius
Mon, Aug 08, 2016 17:24
8

Just a question about disk brake pad wear and brake lever travel...

 

My "best" bike has Shimano RS785 disk brakes, and has now done just over 5600km.  Braking still works perfectly, but I was getting a little paranoid that I've not had to replace the brake pads yet.

I worked out how to get the pads out of the front brake unit and found there was roughly 1-2mm of pad material left - so I don't think they need changing yet.  One thing I have noticed over the last couple of months is that the lever pull for the front brake seems to be a bit longer than it used too (i.e. I can no longer 1 finger brake without my other fingers getting in the way).

It seems obvious that as the pad material wears, the lever travel would increase - is this something others have experienced with these brakes?  I'm not sure at what point I'm meant to replace the pads - I don't particularly fancy adjusting the levers, if when I do replace the pads everything will be out of whack again.

 

Any thoughts?

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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8 comments

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paulrattew | 8 years ago
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If the lever travel has increased substantially then a bleed will almost certainly be necessary - thinning pad will only really effect level travel by the degree to which the pad has worn. Given a brake bleed is a relatively cheap workshop job - if you don't want to do it yourself, which unless you have lots of disc braked bikes to service is a lot of hassle anyway as you waste so much fluid due to it having such a short shelf life once opened -  I would just drop the bike into a shop for them to take a look.

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DaveE128 | 8 years ago
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Check what amount of pad material Shimano recommend you change them at. For my Avids, it's total thickness (including backing) of 3mm I think. I suspect yours might be overdue replacement!

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CXR94Di2 | 8 years ago
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Hydraulic brakes will have further lever travel as the pads wear out. The pistons have to travel further, requiring more lever travel. Change pads for new ones, bleed system with fresh fluid.

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crazy-legs replied to CXR94Di2 | 8 years ago
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CXR94Di2 wrote:

Hydraulic brakes will have further lever travel as the pads wear out. The pistons have to travel further, requiring more lever travel. Change pads for new ones, bleed system with fresh fluid.

No, the whole point of hydro systems is that the feel doesn't change, the levers travel the same distance because the pistons don't retract as much once the pads slim down due to wear.

If the lever is travelling further it's likely there's some air in the system which a quick bleed should fix. Although yes, while you're bleeding it you may as well change the pads too, start with a clean system.

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CXR94Di2 replied to crazy-legs | 8 years ago
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crazy-legs wrote:

CXR94Di2 wrote:

Hydraulic brakes will have further lever travel as the pads wear out. The pistons have to travel further, requiring more lever travel. Change pads for new ones, bleed system with fresh fluid.

No, the whole point of hydro systems is that the feel doesn't change, the levers travel the same distance because the pistons don't retract as much once the pads slim down due to wear.

If the lever is travelling further it's likely there's some air in the system which a quick bleed should fix. Although yes, while you're bleeding it you may as well change the pads too, start with a clean system.

 

The feel doesnt change, but lever travel sometimes will.  The same happens on motorcycles and cars.  I have had an mot failure due to excessive pedal travel due to worn pads.  The pistons sometimes pull back more than they should

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Carton | 8 years ago
1 like

The lever travel shouldn't really change, just pump the brakes quickly several times and they should get back inline.

5,600km without a pad change is almost unheard of, but maybe you're really light and you've been really lucky with the weather. But if there's more than a mm left there's surely no need to change then.

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sergius replied to Carton | 8 years ago
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Carton wrote:

5,600km without a pad change is almost unheard of, but maybe you're really light and you've been really lucky with the weather. 

 

That's why I was getting a little paranoid over it, I'm only 65kg and Dorking where I do most of my riding is hardly the hilliest part of the world - but still it did seem like rather a long way.

I'm not one to drag the brakes a lot and I'm a fairly confident descender so I guess that helps a bit.

 

I did some research on the calipers last night - as mentioned above they are meant to be self-adjusting.  One of the other things I found was a number of people grumbling about the lever pull distance before the brakes engage - it's quite possible I've just not really noticed the issue before (I tend to ride on the hoods a lot).

I don't plan on bleeding the brakes if I can possibly help it... Last time I did that on my MTB was an unmitigated disaster when I didn't RTFM and lost a teeny tinny washer.

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Carton replied to sergius | 8 years ago
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sergius wrote:

I don't plan on bleeding the brakes if I can possibly help it... Last time I did that on my MTB was an unmitigated disaster when I didn't RTFM and lost a teeny tinny washer.

I hate washers as much as much as you do, so take my advice with a grain of salt. But just for context, last time I was in a shop (fouled my mech playing wrench) the mechanic told me my MTB (basically the same caliper as m785s) pads were a bit worn, so I went ahead and had them changed (surprisingly almost as cheap as online). But the lever travel on one lever was completely slack so I asked for a bleed (first one ever, wanted to see it done before trying it at home). The thing was, he had just run out of mineral oil. So he just started pumping it far harder than I ever have, [insert NSFW simile here], and voilà. He did say that if it started bothering me again I'd probably burnt the oil, but thankfully they've been perfect ever since (over a month and 15,000 ft ago).

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