Avoiding air getting into the system on disc brakes

[Nothereforafasttime]

Fairly new to disc brakes but I love them, however one thing bugs me....

Are there any ways to avoid air getting into the system?  For example, I had the bike upside down in the garage the other day to remove the wheels and replace the tyres and when I went to put the rims back on the brakes felt very soft so as a result air had got into the system. So out came the brake bleeding kit...

Is there anything I need to do to try and avoid this from happening again or am I destined to always have this issue everytime I remove the wheels??

Had the bike upside down with the wheels off for about an hour so is this too long a time to allow air to get in?

TIA

[Gattopardo]

Going upside down, you are going to get air so don't put the bike upside down 

Once the bike is back right way up, leave the the brake cable tied on.  Leave over night or longer, magically no more air trapped.

[Nothereforafasttime]

Going upside down, you are going to get air so don't put the bike upside down 

Once the bike is back right way up, leave the the brake cable tied on.  Leave over night or longer, magically no more air trapped.

Thanks for this!  'Leave the brake cable tied on'.  Can you expand on this? 

[simonp]

Rubber band round the brake and bar holding it on.

[grams]

You should also be able to burp any air bubbles that get in this way by repeatedly pumping the brake levers until you feel them stiffen up.

And yes, don’t turn your bike upside down.

[Nothereforafasttime]

You should also be able to burp any air bubbles that get in this way by repeatedly pumping the brake levers until you feel them stiffen up.

I did that after I had put the wheels back on but they soon softened up again

[Gattopardo]

Going upside down, you are going to get air so don't put the bike upside down 

Once the bike is back right way up, leave the the brake cable tied on.  Leave over night or longer, magically no more air trapped.

Thanks for this!  'Leave the brake cable tied on'.  Can you expand on this?

A simon p said.



Motorbike and bicycle brakes are the same...ish

[Gattopardo]

You should also be able to burp any air bubbles that get in this way by repeatedly pumping the brake levers until you feel them stiffen up.

And yes, don’t turn your bike upside down.

Never found that to work for long.

[Ben T]

My tourer which has hope mtb hydraulic brakes gets turned upside down now and again with no ill effect.

I think that some brakes aren't bled that well from the factory. They can have an air bubble but it's above the reservoir and thus the brake will still be firm, but when turned upside down, that air bubble will go into the system and cause sponginess. Pumping and holding on will simply encourage the air back up to above the reservoir, but bleeding it might well eliminate it entirely, meaning if you do turn it upside down again, it might be ok.

That said I do still think it's good practice to not turn it upside down, unless/until you know for sure.

[andrew_s]

'Leave the brake cable tied on'.  Can you expand on this?

Read it as "Leave the brake held on with a cable tie"

[grams]

What are your fluid levels like? It could be you only barely have enough for the system to work.

[Nothereforafasttime]

What are your fluid levels like? It could be you only barely have enough for the system to work.

How do I find out my fluid levels easily?

[Brucey]

What are your fluid levels like? It could be you only barely have enough for the system to work.

How do I find out my fluid levels easily?

You can tell if you have enough fluid by removing the wheel and pumping the brake until the pads touch one another. If the brake is as firm as it is normally, the brake has enough fluid in it for the pads to wear another ~1mm each side. [BTW Don't try this with the pads worn nearly to the backing or if only one piston moves easily; you can push a piston out too far quite easily and this will spoil your day.]

In most modern 'open' systems the reservoir has a rubber diaphragm that moves as the fluid is displaced beneath it, and there need be no airspace. [BTW many motorbikes have MCs that are similar, most cars don't.]  If inverting the bike causes the brakes to go mushy, it just means that the brakes were not correctly assembled and bled; they got all the air out of the pressure lines and MC piston, but left some beneath the rubber diaphragm.

If you have a system that has gone mushy through inversion you can encourage the brake to self bleed by

a) orienting the bike so that the hose run is upwards to the MC and the transfer port between the MC and the reservoir is also favourably oriented. [Usually this means that the lever end of the MC assy is slightly higher than the hose end.]

b) doing the test (with the wheel out) as described above. When you push the pistons back, that usually purges air from the MC.

Obviously this doesn't cure the basic problem, i.e. that there is air beneath the diaphragm. It also doesn't work with some 'road hydraulic' STI type units, because the transfer port is not (ever I think) favourably oriented when the hose run is.

cheers

[D.A.L.E.]

Use cables. Or bleed them properly, like.

[grams]

How do I find out my fluid levels easily?

Are you topping the fluid up as part of your bleed procedure?

(I believe the way to get the recommended level, assuming modern Shimano, is to replace the pads with the yellow bleed block, pump the brakes, then brim the fluid in the funnel thingy)

[Nothereforafasttime]

How do I find out my fluid levels easily?

Are you topping the fluid up as part of your bleed procedure?

(I believe the way to get the recommended level, assuming modern Shimano, is to replace the pads with the yellow bleed block, pump the brakes, then brim the fluid in the funnel thingy)

I'm not bleeding the brakes - my LBS is.  They've sorted it now so I think the best thing I can do by the sound of most of the replies here is not turn my bike upside down if I need to remove the wheels (tricky I guess if you're riding on your own and there's no logical place to hang the bike the right way up.  Always turned other bike upside down but these had caliper brakes so not an issue). 

My system is a Shimano Ultegra with STi levers.

Note to self - learn how to bleed your own brakes!

[Samuel D]

They've sorted it now so I think the best thing I can do by the sound of most of the replies here is not turn my bike upside down if I need to remove the wheels (tricky I guess if you're riding on your own and there's no logical place to hang the bike the right way up.  Always turned other bike upside down but these had caliper brakes so not an issue).

You could, and probably should, lay it on its side. Gently so as not to scratch the saddle and handlebars.

https://www.sheldonbrown.com/brandt/upside-down.html

[Hot Flatus]

How do I find out my fluid levels easily?

Are you topping the fluid up as part of your bleed procedure?

(I believe the way to get the recommended level, assuming modern Shimano, is to replace the pads with the yellow bleed block, pump the brakes, then brim the fluid in the funnel thingy)

I'm not bleeding the brakes - my LBS is.  They've sorted it now so I think the best thing I can do by the sound of most of the replies here is not turn my bike upside down if I need to remove the wheels (tricky I guess if you're riding on your own and there's no logical place to hang the bike the right way up.  Always turned other bike upside down but these had caliper brakes so not an issue). 

My system is a Shimano Ultegra with STi levers.

Note to self - learn how to bleed your own brakes!

You need a bleed kit with funnel, some Shimano mineral oil, and an internet connection to look for YouTube videos.

It is very straightforward, but it pays to be fastidious, cleaning ports and caps with isopropyl alcohol first and being very disciplined with avoiding contaminating pads.

Incidentally, I'm not sure about newer hydro discs but mine (The first di2 hydro levers) had a lot of travel, which I don't like. Every couple of weeks I pop the wheels out (Just enough for caliper to clear disc) and give the lever a couple of pumps. It pushes the pistons out and reduces the travel. If you over-do it you have to push the pistons back in with a tyre lever.

[Ben T]

You need a bleed kit with funnel, some Shimano mineral oil, and an internet connection to look for YouTube videos.

It is very straightforward, but it pays to be fastidious, cleaning ports and caps with isopropyl alcohol first and being very disciplined with avoiding contaminating pads.

And possibly the correct funnel adapter. Don't necessarily assume the "shimano" bleed kit comes with the correct funnel adapter. e.g. R8070 needs this one https://www.sjscycles.co.uk/gear-spares/shimano-duraace-str9120-funnel-adapter-oring-y0c698050/

It is easy though once you've got the correct tools, watched the video a few times and set it up correctly.

Every couple of weeks I pop the wheels out (Just enough for caliper to clear disc) and give the lever a couple of pumps. It pushes the pistons out and reduces the travel. If you over-do it you have to push the pistons back in with a tyre lever.

Interesting tactic, does it not make it so that it drags when you put the wheel back in?

[Hot Flatus]

Funnily enough, no. Couple of pumps, then wheel back in, repeat if necessary. If it rubs, you just push the pistons back in. It's the only way I've found of keeping the brakes firm.

[Ben T]

Hmm. Must be a balancing act when designing/making brakes, too much drag on one hand, too much travel the other. Shimano have definitely erred on the side of not having drag, at the mercy of there sometimes being too much travel.
Hope mtb brakes can be the other way, new pads sometimes drag - but a trick is, if you put some very thin plastic or even card either side of the disc to make it as if the disc was slightly thicker and give it few pumps with it like that, then when you take it away and have the disc on its own it will be perfect.

[rafletcher]

You don't get much drag (as has been mentioned elsewhere before) because the run-out on the disc (the wobble if you like, it'll have some) knocks the pads back sufficiently to reduce it to negligible levels.

[Brucey]

Hydraulic brakes are always designed to retract the pistons. It is fairly obvious that as the MC is released, the hydraulic pressure drops below ambient and atmospheric pressure will push the pistons back, at least until the compensation port in the MC opens (in an open system).  This provides for gross piston movements and provides the self-adjusting characteristic. It of course doesn't work perfectly if there is any air in the hydraulic system, since there is more MC movement required to generate the low pressure and the lowest pressure won't be as low this way.

What is less obvious is that the seals in the caliper body are designed to deflect and/or rotate slightly, and (provided the seals are in good condition, and the pistons are not binding, almost regardless of what is happening in the rest of the hydraulic system ) this always gives a small measure of piston retraction.  In a well designed system, the piston retraction so obtained is enough to allow the pads to clear a disc that is in spec for runout. Note that the pads may be 'knocked back' (if there is no spring, eg on a motorcycle brake) but the pistons are not knocked back in the same way; they are retracted. If the pistons don't self-retract properly and are causing the high spots on the disc to contact the pads, this soon causes runout induced DTV (Disc thickness variation) in most vehicle brakes and this turns the brakes to junk. It is not an acceptable condition.

The seal design that provides retraction is most commonly (and is this way in all shimano calipers that I have stripped down) a square section seal that is in a groove that is not quite square; on the air side of the seal the groove has an angled face, so that the seal can deform and the piston can move slightly without sliding through the seal. Hydraulic pressure on the back of the seal deforms it into a parallelogram shape when the brake is on.  When the seal recovers its square shape, the piston is retracted.

If the pistons don't retract 'enough' this can be because

- the disc isn't within spec for runout
- the hub bearings are slack
- the seal groove is filled with crud (typically corrosion product) on the air side
- the pistons are not free-running in the caliper bores
- the seal is contaminated and swollen so that it can't move in the same way as normal.

Both the last two are very common; all kinds of 'normal stuff' that you might use on your bike can result in the pistons (which are usually plastic) and/or the seals (which vary depending on who made them and what fluid is meant to be inside the hydraulics)  swelling up.

The caliper bodies are usually machined after being surface finished on the outside, which means that the caliper bores are usually unprotected aluminium. If the bike sees weather, the bores usually start to corrode. This corrosion is x10 or x100 faster if there is road salt present. The corrosion is worst in the air side of the  seal groove, and can completely prevent the usual seal rotation and therefore piston/pad self-retraction. Given a little more time, such calipers will fail to a leak at the piston seal, and the corrosion is no longer confined to the air side of the seal alone, and creeps under the seal itself.

If you are sure that no contaminants that might cause swelling have been near the pistons and seals, you can take it as read that non-retracting pistons is a good sign than the caliper may fail to a leak anytime soon because the piston bores/seal grooves are corroding.  If you are adventurous you can strip and fix the caliper  (the corrosion damage doesn't always damage the caliper beyond repair) but most folk just replace it with a new one.

BTW with Hope brakes (which use DOT fluid inside) they recommend that you use a silicone spray lubricant to help prevent water ingress into the piston bores. It is not clear to me that this product is 100% safe to use on mineral oil brakes; there is a chance that the solvent in it attacks the seals in mineral oil brakes, and may cause swelling.

[Genosse Brymbo]

I have a passing interest in hydraulic discs because I believe bikes I may buy in future could well have them.  I'm a bear of little brane and can't work out from this thread whether the OP's problem is resolved by not turning the bike upside down or by correctly bleeding the brakes.  Are bikes with hydraulic discs meant to remain upright only?  If so, does this mean they can't be hung from the meathooks on trains (Virgin and the Caledonian Sleeper come to mind)?  Also, what's the situation with regard to air transport of the bike?

[rogerzilla]

Some brake systems have no air in them at all once bled and shouldn't do this (e.g. Avid Elixir, but this doesn't work because as soon as the volume of fluid starts to increase through absorption of moisture or even heat from prolonged braking, the brakes start to drag because there is no room for expansion). 

Are we talking about the type with a small reservoir and a bellows (so it can be sealed, unlike a car's brakes where the reservoir can breathe) somewhere near the lever?