Disc Brake Problem

[podgeskeeper]

Hello All,

I have a set of Shimano BR-M446's and the front brake lever comes almost all the way back to the grip. I've checked there's no air in the system and the pads are no where near worn. Has anyone any ideas about adjustment, is it possible to adjust disc brakes? The rear brake is fine, I don't understand why there's a difference.

Any and all help gratefully received.

James

[Brucey]

this kind of trouble is nearly always because the brake needs to be bled.

Service instructions here;

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/SI_8J50A/SI-8J50A-002-00-Eng_v1_m56577569830747469.pdf

caliper/disc parts listing here;

http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/BR/EV-BR-M446-R-3227_v1_m56577569830753728.pdf

master cylinder parts here;

http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/BL/EV-BL-M445-2994_v1_m56577569830702224.pdf

bleeding instructions (NB yours may be different to this needing the funnel adaptor etc);

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Bleeding_SI/SI_0037A/SI_0037A_002_EN_v1_m56577569830648123.PDF

On any hydraulic brake system it is possible for the MC seals to fail; one mode of failure leaves the brake with a mushy feel and a long lever travel. You will note that shimano don't supply seal kits for their master cylinders so it is a new lever assy in that case. Obviously try bleeding the brakes out first, several times, and the other suggestions in the SI before reaching that conclusion.

hth

cheers

[podgeskeeper]

Thanks brucey for this comprehensive reply. Will give a complete bleeding a try.

Thanks for your help.

James

[guillaumez]

Hi,

I have a slightly different problem with my M445 brake set. I have ridgeback flight 02 from (nov 2010) which original brakes pads and disc that lasted nearly 18 months and 3000km.
At this point I replaced the sets of pads at my LBS and within 2 weeks I had lost all breaking power in the rear. The front one was great until from one day to the next I lost all at the front as well (that was only 2 months since replacing the pads). All brakes squeaking crazy even when just pushing the bike. Went back to my friendly LBS and investigation started.

Brake pads were not worm and after cleaning they were still not working (I think they may have been metallic pads).
A new set of brake pads were fitted but again impossible to brake safely
System was flushed, refilled etc and a new set of resin B01S were fitted.

All seemed working much better (although not as good as when I first had the bike) but after about a month the rear brake became useless (and squeaky) and a month later the front one has now gone too.

The discs gets fairly warm when I brake (>50C) for a few seconds but I have no comparison so I don't know if it is actually too hot or not

I am returning to my LBS to try to figure out what's going on but it was already puzzling us last time.

Could the disc themselves be knackered?

Thanks for your help

[Brucey]

Friction couples are a dark art; discs become 'conditioned' with certain pads and then don't always work perfectly if different pads are used unless the new pads are themselves abrasive and wear the disc surface.

However in your case I am suspicious of contamination being the cause of your troubles. The slightest amount of grease or oil on the discs or pads will stop the brakes for working as well as they did before. Fingerprints (from clean hands even) are a cause for concern.

Possible sources of contamination include;

-chain lube
-bike cleaning products
-polishes
-contaminated road spray (diesel or oil from the road surface)
-brake fluid leak
-too much anti-sieze compound on the pad backings

once pads have been contaminated with oil, that is it, you have to chuck them away. It doesn't matter how well you try and degrease them you will never get it all out, and the brakes will never work as well as they should. Very often the discs are impossible to clean perfectly also, even though it looks like smooth shiny metal, it isn't...

The disc braking surface will look like a ploughed field under a microscope and that surface will hold oil. Worse, the disc perforations are usually plasma or laser cut; the cut edges are even rougher and can hold even more contaminant. Car or motorcycle discs with light contamination can sometimes be rescued by hard braking which allows the discs to heat up to the point at which the contamination simply burns off. You can try and do this on a bike but it is potentially dangerous; you need a big hill and the discs may warp in the process.

A brake fluid leak in a Dot 4 system will manifest itself because the brake fluid will ruin any paintwork it comes into contact with. However some brakes use mineral oil, and if this leaks a tiny bit then the brakes just don't work very well. New pads may provide temporary relief until they too become contaminated.

hth

cheers

[guillaumez]

Thanks Brucey for the reply.

I think I will try to replace discs and pads and take it from there. Those are cheap replacement parts so it may be time for the discs to go. I'll do a microscopy comparison betwen the old and new for the sake of it (as I can do it at work) this could be fun.

There was some new development this morning. On a wet and much cooler morning the brakes were working much better. Whereas I couldn't stop last night, this morning was much safer .

[Brucey]

There was some new development this morning. On a wet and much cooler morning the brakes were working much better. Whereas I couldn't stop last night, this morning was much safer .

that is interesting; when it is wet some pads and discs both wear at an accelerated rate, and this can mitigate the effects of light contamination to some extent.

I'd be very interested to see the results of any microscopy work. SEM will show the disc surface best, but SEM work on contaminated disc and pad surfaces is problematic; no-one wants the contaminants in their vacuum system.

cheers

[SimonCT]

If they work better when wet its almost certainly oil on the pads. Give them a squirt from a water bottle then try them.
Sintered pads can usually be degreased by gently heating over a gas flame until the smoke stops (Resin pads just pop and separate from the backing). This is just what I do - you might not like it!
Very light contamination of pads can sometimes be shifted by getting them good and hot down a hill.
I've read (but not tried) that cleaning discs and pads in a dishwasher might work.

[guillaumez]

I'll give the discs an acetone wipe before going home tonight.

I probably won't have time for the pads but overall I have an inherent problem recurring every couple of months so I don't mind starting with fresh discs and pads ate they are cheap consumables.

[guillaumez]

Ok I had discs and pads replaced 2 weeks ago. Replaced M445 discs for some XT ones with shimano pads. After 3 days and less than 30km the rear brake starting sqeeking gradually getting worse and at the same time I was loosing brake power. This was followed by the front one. Under heavy rain (Monday/tuesday) this was fine and brake power was restored to an acceptable level. Once the rain stopped the noise returned and has become embarrassing and I can hardly stop (I slow down but not stop). All that in less than 100k.

This only leaves a tiny brake fluid contaminating the pads??

[Brucey]

when you say squeaking, do you mean during braking, or all the time? Is the lever still mushy at all?

If it is another 'contamination episode' it may prove difficult to prove beyond doubt where the contamination is coming from. If it is a tiny caliper leak, there should be signs of fluid (oil, I think, in your system) behind the pads, that, once cleaned will return in a short while (a day or two). But it doesn't need much, and it may be difficult to spot. Since there are numerous other potential contamination sources, it is difficult to exclude them all.

Sometimes you can smell what the contaminant is; maybe it is worth having a good sniff!

BTW for hydro brake systems that are used for long descents, the use of sintered pads isn't necessarily a good idea, because more heat (too much) gets into the calipers. But if you don't do big downhills, maybe they are a good choice, simply because it seems you have a fighting chance of cleaning them should they become contaminated.

cheers

[guillaumez]

Hi,

the squeaking is during breaking but at low speed (<10-15kph). When you first decellerate there is no noise because friction is too low. Brake levers feel normal: not hard to press and short travel to get pads pressure.

I'll have a look if I spot anything around the calipers.

This is on a commute bike, route is fairly flat.

[Brucey]

It does sound like contamination; if you can take the pads out and look behind them this is where you will best see a fluid leak

cheers

[irc]

Can't understand all these problems. Any time I mention simplicity as an advantage of V brakes someone says disc brakes are also trouble free and just as easy to keep runnning

[reohn2]

Can't understand all these problems. Any time I mention simplicity as an advantage of V brakes someone says disc brakes are also trouble free and just as easy to keep runnning

They are,if they're BB7 cable discs