26
What you experienced was brake fade. There are two reasons why it happens:
The fluid inside the brake boils releasing gas into the system which makes the brakes feel spongy and then non-existant.
The pad and rotor heat up sufficiently to not work, this feels like you are applying the brake but it doesn't work.
You look to have experienced the first. This ...
22
First of all you need to tell the LBS (local bike shop) that the disk can be "fixed". Then you need to find another LBS because they are either amateurs or are simply trying to make you buy stuff that you don't need.
You need to remove the oily disk from the bike and use a bike degreaser or alcohol on it to remove all oil. Rub with a clean dry cloth or ...
16
There are two risks to turning your hydraulic brakes upside-down.
The brake system isn't filled to overflowing with hydraulic fluid: there's likely to be a small air bubble. Normally, this sits at the highest point of the system: the oil reservoir at the brake lever. There, it's not compressed by the piston when you brake, so it can't interfere with braking....
16
Yes. Disc brake rotors can warp a little in use, or get bent from impacts and straightening them is routine maintenance.
The basic procedure is to find spots where the rotor is bent inboard or outboard, and carefully incrementally bend it back. There are tools made for this purpose (e.g. Park tool DT-2) but the job can be done with a large adjustable wrench....
15
Usually what you do is disconnect it at the lever, install the lever to the bar, and run the hose up from the caliper end. Because it's sealed on the other end, no more than a few drops of fluid will escape. Then once it's through you can size the hose by turning the bars as far as they go while holding the hose up to the lever and finding the length where ...
15
If there are no signs of a metallic wire (often colored silver) exposed anywhere near either end of the black housing "tube" connecting the lever on the handlebars and the caliper on the fork, it means that it is a fully closed hydraulic line. Another giveaway for mechanical brakes is a moving arm at the caliper which visibly reacts when pressing a lever.
...
15
The Older Paradigm of Women's Specific Bikes
Argenti and Kaz both are correct, but let's dive a bit deeper into women's specific design (WSD). Their answers allude to an old paradigm of WSD. It appears to be based on anthropometric measurements collected by the US Army around the 1950s. Here is a relevant 2017 interview with Stephanie Kaplan, a product ...
15
Providing the 7900 is the same, I’m sure it is but someone more qualified can chip in if it’s different.
How did it fall out ? Shimano have a proper tool for removing them.
Info from here Fawkes
13
Cable discs rule if you are riding a lot of third world. I am a million miles away from a shop, access to the oil, etc. A hydraulic failure would be disaster, whereas the worst thing that will happen with a cable disc is a lose bolt/need for readjustment. As smooth as hydraulics are, the fix of a failure when in the middle of nowhere (whether third world or ...
12
No, the thermal conductivity of brake hose/fluid is way too low.
I've run the numbers on this -- very roughly to make the calculations simpler. I've assumed the hose isn't cooled by airflow for some reason and a solid hose or equivalently that the fluid has the same thermal conductivity. Even the lever is assumed not to lose any heat to the air. All ...
12
Originally I gave a tentative answer to this question and proposed the solution without actually trying it (next three paragraphs), and now that I have tried it I expand the answer with a report on the results (last two paragraphs). Spoiler: as already verifed in the meantime, the solution works perfectly.
Firstly, this is not a bleeding problem. I verified ...
11
The least you need to do is bleed the brakes to remove trapped air. It's best though if you completelly change the fluids and bleed. You can do that in an LBS, or yourself if you have a bleeding kit and know how to do it.
The reason for the behavior you described is that air is trapped in the system and makes things mushy and unpredictable. The fact that ...
11
The comments OraNob and Batman said are spot on. A cable cutter uses shear force (focused crushing) to cut through the metal wound wires in a cable housing but still requires some clean up because of this. This is why you need to go back and reform the inside wires with an awl and clean up the edges with a file or sanding pad. A plastic hydraulic line can ...
11
I highly doubt you'll be able to find a separate alloy casting to replace the one that broke. Even if you can you'll be faced with assembling the caliper which is not trivial.
Just buy a replacement caliper. Make sure you refill/bleed the hydraulic fluid according to Shimano's documented procedure, of have a competent bike shop do it for you.
10
Here are a couple of downsides of both types, that I have witnessed.
Hydraulic. Leaking oil tends to get into the pads and rotor, rendering the first useless and the second in need of professional cleaning.
Mechanical. Cable failure is very perilous. Essentially one moment you have full stopping power, the next moment the brake is disengaged fully. ...
10
Most likely cause if the disc and pads are contaminated. What did you wash it with? Many cleaners leave a residue.
Worst case the pads need replacing. The discs should be well cleaned before installing new pads - Use a solvent such as brake cleaner, methylated spirits or Isoprop alcohol, and rub the discs to be certain no contamination is left on them.
...
10
I believe it is not worth trying it. You risk contaminating, scratching, or bending your rotors, as well as scratching or cracking a brake pad.
Consider also whether there are bigger problems with your bike setup if it's too much hassle to remove your rear wheel to change your brake pads — how often does that happen: every 2–3 thousand kilometres? Is there ...
10
I've had my fair share of hydraulic brake woes this year so I'll list the things I'd check - I am aware that you've done some already. Firstly some questions though:
Are these brakes using mineral oil or dot fluid? If it's dot fluid, when attempting to bleed the system you may want to take extra care and de-gas the liquid like with Avid brakes.
The comments ...
10
A few years ago this was a reasonably common setup as sold. I've certainly ridden an e-bike set up that way, with v-brakes at the back and a mechanical disc brake at the front. I suspect this wasn't so much about stopping power as not needing maintenance between services, though in practice some adjustment was required even if less than at the back.
10
Rotors can and do bend, subtly. Have you checked it's flat? One of mine sounds flat (or rather doesn't sound) cold, but has a high enough spot when hot to sing.
Have you checked for play in your wheel bearings? That can cause rotors to touch especially when cornering.
Even if the rim is a little out of true, that won't affect the rotor, which is rigidly ...
9
I would try repairing them before binning them and purchasing the Shimano upgrade. If there is no obvious fluid leak around the piston the seals are likely in good order and you've got a sticky piston rather than a failed calliper. This is a simple fix:
Remove wheel and brake pads.
Using a flat headed item hold the working piston in.
Squeeze the lever, the ...
9
From Shimano Service Instructions:
When turning the bicycle upside down or on its side the brake system may have some air
bubbles inside the reservoir tank which are still there when the reservoir tank cover is
replaced, or which accumulate in various parts of the brake system when it is used for long
periods. The M755 disc brake system is not ...
9
Bedding-in is a common process with new disc brake pads. Disc brakes were developed much earlier for motorcycles and cars, from which the technology and the terminology has been transferred to MTBs: see https://ebcbrakes.com/articles/motorcycle-bed-in/ and http://www.centricparts.com/files/White%20Paper%20Revisions%204-2012/Centric%20White%20Paper%20C3-2012-...
9
As mentioned in comments you most likely applied the brake without the disc between the pads. The pistons advance and do not return all the way to their original position - this is how hydraulic brakes automatically adjust for pad wear.
The pistons can be pushed back in. Remove the pads then use a tire lever to push the pistons back in. Then replace the ...
9
TL;DNR - single finger levers have little (if any) advanatage and some disadvantages.
Most people do not start out cycling with XT/Ultegra quality bikes and therefore (do not) start out on bikes that have brakes good enough for one finger control. Even today, many low end disc brakes cannot reach full braking potential (i.e. wheel lock), with one finger. ...
9
It is a brake - a critical safety component. Just replace it.
Any fix or hack or bodge could fail you at the worst moment, endangering you and others around you. This one has already broken, who's to say the rest of the casting is any better?
If it helps the acceptance process, find the cheapest brake caliper that you would accept. This is your Sunk cost,...
9
The part that popped out is one of the pistons.
There should always be a bleed block in place between the pistons when bleeding the brake. Immobilizing them this way is what prevents overfill and also prevents the pistons from popping out if you go to squeeze the lever as part of part of the bleed procedure, both of which happened here.
Excessively narrow ...
9
pumping worked to no avail-which usually works
This is ringing alarm bells for me - it implies you encounter brake fade on a semi regular basis.
This is almost certainly down to poor braking technique (dragging the brakes excessively). You need to learn to let the bike pick up speed and then brake hard for a short period and repeat. This approach gives ...
8
I store my bicycles this way for the summer (winter fat tyre bikes) and winter (non-commuter mountain and road bikes). I haven't ever had any issues with this and the shop I worked at previously stored all the most expensive bikes this way as well (less expensive bikes were warehoused). I haven't ever had any issues where my brakes suddenly didn't work ...
8
The official answer from Shimano as of now is no, as per their road hydraulic systems compatibility chart.
Someday there may be flatbar road hydraulic levers that make their way onto a new version of that chart, but for now bikes like that just come with conventional MTB/hybrid systems.
There may be combinations that work acceptably, but they will be hacks ...
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