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What are the possible on-road modes of failure for hydraulic bicycle braking systems, and what can a touring cyclist do to be prepared for their eventuality? What can one carry as an emergency backup and when might it be needed?

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    Consider modifying the question if you find these follow ups relevant: what region of the world, e.g. West or less-resourced country? How long are you touring? How much space and weight do you have available for spares?
    – Weiwen Ng
    Dec 28, 2022 at 18:03
  • This is why mechanical disk breaks exist.
    – Puck
    Dec 29, 2022 at 13:15
  • The conclusion one draws from reading the many answers here is that hydraulic brakes may not make sense on a touring bike. To read a contrast with why hydraulic brakes might make sense on an MTB, see bicycles.stackexchange.com/q/82473/48599.
    – Sam7919
    Dec 29, 2022 at 15:52
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    @Sam Real loaded mountain descents remain best served by rim brakes. There's not a lot any disc brake can ever hope to do to manage heat as well. Dec 30, 2022 at 2:16
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    @NathanKnutson what? Downhill/enduro discs are plenty capable enough for even the heaviest-loaded road descents. Discs dissipate heat just fine (otherwise they wouldn't be used in motor vehicles); it's just that road-targeted ones are usually undersized to save weight. Dec 30, 2022 at 22:33

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There's really nothing one can carry to repair hydraulic brakes, short of workshop tools.

Instead, do your maintenance well before leaving on a long trip, and have a couple of short bed-in rides before packing up.

Also, carry a phone to call for help, money/card for a bus/taxi/etc and know where the bike shops are on or near your intended route. If touring across holidays also note their hours and which days they will be closed. Some prior knowledge helps.

Finally do consider that your brakes are two independent systems, and most of the first world legislates something to this effect. So if one brake breaks you still have a second functioning brake to carefully continue through to a LBS for repair.

I wouldn't ride down any significant grade with only one working brake, but I'd definitely ride a flat route to a LBS with gentle braking, coasting up to stops, and start braking twice as early Maybe more if I only have a rear brake.

Expect to pay extra for emergency/urgent repairs at a shop, but that's just how it is.

This is one reason why hard-core touring types prefer cable-based brakes.

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The one field repair to Hydraulics I have ever had to do required removing the caliper and riding without a rear brake (A couple of hours technical single track MTBing without a rear brake is somewhat 'educational'). The return valve in lever blocked (fluid went in, but not out of the reservoir), meaning the brake once applied would not release. Required for that repair was a Hex key and zip ties (to strap the caliper to the seat stay)

Most of the failure modes are common to cable brakes (contamination, bent rotor, broken lever) with the same resolution. The only failure modes that are hydraulic specific and a complete failure of the brake are fortunately rare (like mine). Even the need for a bleed is usually not total loss of braking. It typically results in spongy brakes that need pumping and have lost the fine control needed for max control, but you can still get near 100% braking force.

The smallest and lightest solution that would allow a field repair of hydraulics, that covers the half dozen most common failure modes, would be a spare set, pre bleed with cable ties for the hose.

Hydraulic specific failure modes I can think of

  1. Stuck Piston - annoying but rideable - preventable with maintenance.
  2. Burst hose (hose failure)- No braking, hose + bleed, uncommon.
  3. Mechanical damage to hose -No braking, hose + bleed - not common, crash damage, shipping, not common notmal riding.
  4. Lever internals stuck/jammed/not working - new lever and bleed, very uncommon
  5. Caliper leaks - Brakes probably working for a while, limp home usually possible. Caliper overall, new seals, bleed
  6. Crash damage to caliper/lever including pulling out banjoes/hose end - replace broken bits and bleed.

As you can see, nothing here involves a bleed without other parts being required. No point carrying a bleed kit alone.

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    If you are going to carry a spare brakeset, I would recommend SRAM (shockingly enough) because their levers are ambidextrous and the fluid is readily available. Most other brands have left/right-specific levers and/or use a special brake fluid.
    – MaplePanda
    Dec 29, 2022 at 0:22
  • @MaplePanda Mineral oil is also readily available in the form of baby oil at any pharmacy :) Works great, but it does void your warranty to be fair.
    – Andrew
    Dec 29, 2022 at 18:25
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    @Andrew Fair point, although I'll leave the question of "is baby oil safe" for individuals to decide upon. Also, Shimano brakes still have the issue of left-right specificity.
    – MaplePanda
    Dec 29, 2022 at 20:01
  • @MaplePanda This refers to the flat-bar levers, right? Mar 29, 2023 at 12:13
  • @VladimirFГероямслава Ah yes, I primarily had flat bar levers in mind while writing that. Although now that I think of it, I wonder if you could reprogram SRAM eTap levers to control the FD with the right shifter and the RD with the left, effectively making them ambidextrous as well.
    – MaplePanda
    Mar 29, 2023 at 19:59
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Fiona Kolbinger won the 2019 Transcontinental Race, an endurance race across much of Europe. She was reportedly on a 2019 Canyon Endurace, which I believe had hydraulic discs. She finished in about 10 days. In her reported loadout, she had spare disc pads (bottom left of the photo), but no other spares.

Bikepacking.com wrote in 2021:

Cable brakes are easy to see and understand, so almost anyone can get their head around them. Hydros require more knowledge and pre-trip experience with how they work. I think hydros are super reliable most of the time, but if one does fail for one reason or another, it’s difficult to do much about it, compared with carrying a spare steel cable.

They also suggested that a bleed kit would probably be worth packing only for longer trips.

I did not see mention of carrying spare rotors. I have no experience touring. I understand that some circumstances can warp the rotor - those are likely to be prolonged descents with heavy braking. So, depending on the terrain and your load, it may be worth considering spare rotors.

Riders should probably consider the possible spare parts situation in their operating area. The Transcontinental race may have been in an area where bike shops were relatively easy to get to, and many of them might have had servicing equipment for hydraulic brakes. In other countries, only shops in the larger cities might be able to service hydraulic brakes. So, your operating area might bias you toward cable discs or cable-actuated hydraulic brakes, and perhaps you can consider packing spare cable and housing.

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The only field repairs I can imagine making on disc brakes are:

  • Straightening a slightly bent rotor. There are "tuning fork" tools for this purposes, some compact multi-tools include these.
  • Replacing worn-out pads. This is straightforward and can be done using tools you're probably carrying anyhow.

If you had a rotor bent so badly that you couldn't rotate the wheel, you could in theory carry a locknut removal tool to remove the rotor, remove the brake pads and hope that gives you enough clearance (note: you could mess up that caliper or rotor if you accidentally pulled the lever with no pad), or remove the caliper from its mount and zip-tie it to the frame/fork so it's out of the way. Obviously these solutions put you in a "limp home" situation with one brake.

Note that there's nothing specific to hydraulic brakes here—you could do the same with cable-actuated discs. It's hard for me to imagine bleeding the brakes by the side of the road.

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  • As a corollary, there exist ultralight/portable lockring tools for centerlock rotors. May be worth checking out if one intends on carrying a rotor removal tool.
    – MaplePanda
    Dec 29, 2022 at 0:17
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    @MaplePanda I thought about that. I’m not sure if those portable lockring tools will work on the brake side due to the spacing between lockring and frame.
    – Adam Rice
    Dec 29, 2022 at 2:21
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The possible on-the-road failures cover a wide swath of issues and the answer for most of them is there's not very much you can do. One can contrive situations where any of the hydraulic system parts fail and they need to be replaced and the system re-bled. Generally there won't be a practical way of carrying what you need to cover those scenarios.

Of the reasonably likely failure scenarios, probably the main ones are the brake gets too hot in a loaded mountain descent and the fluid boils, or some kind of impact or snag damages a line (happens easier on loaded bikes). Both require a bleed and possibly parts replacement.

To be prepared you would need a bleed kit, oil, and then whatever amount of redundant parts you want. Also, doing those kinds of repairs in an improvised setting is advanced; simply having the hardware is far from all that's needed. It's possible to imagine having some kind of stripped-down bleed kit with smaller-than-shop-size syringes, a minimalist Shimano funnel if applicable, etc, and then being able recover from an issue in the middle of nowhere, but it doesn't sound very realistic for most.

Fixing broken hydraulic brakes by definition requires having the means to take something that has oil on it but shouldn't and make it clean and residue-free. Often you could make do with a minimal amount of 95% alcohol, say on wipes in a foil packet, but sometimes you'll need more cleaning power, i.e. more serious solvents. (It's true that automotive brake cleaner can usually suffice here and is fairly obtainable if you're near civilization).

There are limits to what a categorical answer can cover, because the details of the specific brake in question has bearing on how successfully one could fix it in various low-resource situations. For example, the Shimano road hydros really do need the part of the bleed procedure abided by where the lever bleed port is turned front and back at 45 degree angles while the lever is compressed to work all the air out. That pretty much necessitates having the funnel and extension tube, or somehow finding a way to substitute them. (Possible, but sounds difficult to improvise out of nothing while camping in the rain etc). To contrast, older generation Shimano flat bar levers with the open reservoir are exceedingly tool-minimal to bleed, since you don't technically need to do anything to the lever end other than open it up, and the caliper end needs only the syringe and hose.

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All the failure modes for a hydraulic braking system on a bike fall into one of two categories:

  • Hydraulic-specific failures.
  • Non-hydraulic failures.

Pretty much all the hydraulic-specific failure modes involve loss of integrity of the hydraulic system, which means that fixing them right requires topping-up and bleeding the system on top of whatever else is needed as a fix. This is not easy to do in the field (in fact, it’s not easy to do in a garage unless you have a work stand), and requires carrying a bleed kit (these can be pretty lightweight, but are single-purpose) and spare brake fluid (either mineral oil or DOT fluid as appropriate, both are actually pretty dense and thus heavy). You can in theory work around this by carrying a full set of pre-bled replacement brakes, but that may be too heavy (and too awkward to pack) for your tastes. However, provided you cycle carefully and properly maintain your brakes, none of these failure modes are likely to happen unless you crash (and at that point, you probably have more than just your brakes to worry about).

The non-hydraulic failure modes are pretty much all shared with whatever the underlying braking technology is. For the common case of hydraulic disk brakes, you’ve got all the disk brake specific failure modes to consider, but pretty much all of these are easy to plan for and easy to fix in the field:

  • Bent/warped rotors: Carry either a rotor truing tool or a pair of parallel-jaw pliers with long, narrow jaws, either of which can be used to deal with slight bends and warping in the field. Possibly carry a spare rotor or two plus the tools required to replace it (though note that rotors that are that badly bent are rare unless you crash).
  • Damaged/worn-out/contaminated pads: Carry spare pads and whatever tools you need to replace the pads. Usually you will have all the tools you need in your existing kit, but you may want to pick up a dedicated pad-spreader or piston press to reset the pistons properly when you change the pads.
  • Contaminated rotors: Carry a small bottle of appropriate cleaning fluid and other required cleaning materials. Make sure to pick up any used materials to dispose of properly later.

Irrespective of all of this, pretty much all modern hydraulic brake systems on bikes are split (you have completely independent hydraulics for the front and back brakes, unlike the norm in cars where it’s a single system for all four wheels). This means that one failing should not render the bike unrideable, you just need to prioritize getting somewhere that can fix it properly and cycle extra cautiously (don’t go down steep inclines, coast to as stop when possible, etc).

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Besides the ever bending rotors and pads eroding like a butter under the sun, the most horrible thing you'll encounter is lazy pistons.

Hydraulic disc brakes are not fit for long bike touring. Calipers will take a lot of beating from mud contamination, dust, water and sand. Calipers on motor vehicles are properly isolated and made, unlike bike calipers which are made to be light and compact. The rectangular o ring that is responsible for retracting pistons is too exposed. That's the primary reason for brake rub, on motor vehicles there is extra sealing like rubber boots.

Cleaning piston area can be done as a roadside repair, but depending on terrain you ride, pistons may scratch badly due to fine sand ingress which cause leaks, or damaged o rings. Shimano for example, does not sell replacement pistons or rings, even so, after replacing those, which can be done by a multi tool, will require bleeding and refill. You need to carry lots of tools, which is not possible. A bare set of tools require a few liters of space, not to mention carrying messy items like oils which can spill.

For hydraulic disc brake benefits like strong brakes with minimum effort, I suggest using cable operated hydraulic brakes at least, which reduces the amount of things that will fail.

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    MTB calipers can take a lot of beating from mud contamination, dust, water and sand before developing serious problems. They're certainly more resilient than any mechanical brakes. Dec 30, 2022 at 22:45

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