Anatomy of a failed hydro caliper

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Brucey
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Joined: 4 Jan 2012, 6:25pm

Anatomy of a failed hydro caliper

Post by Brucey »

had this one in the 'pending' heap for a little while; a shimano BR-M447 hydraulic caliper that started to leak really badly after a winter or two's use.

The pics tell their own story really

caliper in kit form
caliper in kit form


crusty corrosion in seal groove
crusty corrosion in seal groove


more crusty corrosion in the other seal groove
more crusty corrosion in the other seal groove


recess for caliper half seal
recess for caliper half seal


piston seals
piston seals


Good points include

1. that the drilling into the piston chamber is slotted, which means the piston chambers should bleed out OK over a fair range of mounting angles

2. That the seal and seal groove geometry apes that found in car brakes, giving the pistons an automatic retraction to give running clearance.

3. That the piston bores are offset on the pads, so that the pads don't wear too unevenly

4. one half of the caliper body is well polished where the body half seal bears

Bad points include that

5. The body half O-ring may not seal, being made of a rubber that has gone hard and seated in a badly machined pocket.

6. The piston bores and seal grooves are bare aluminium and will corrode very easily with even the slightest winter road salt. You can see the difference in appearance of the exposed bore vs the bore on the wet side. You can also see some filiform corrosion beneath the paint in one of the pictures.

7. The main source of leakage was around the outside of the seals, because of a thick layer of corrosion in the OD of the seal groove.

8. The mineral oil didn't seem to stem the rate of corrosion much

I would say that such brakes are inevitably going to spring a leak at this point, if they are used in winter weather; expecting a seal to carry on sealing against a bare aluminium groove is 'rather optimistic'; salt water will chomp into it like nobody's business.

You can just about see that the seals themselves are wrinkled slightly, where they have been pushed up against the corroded parts of the groove. Nonetheless this caliper may live again, provided the seal groove isn't found to be badly pitted once it is cleaned up.

On balance I think that (for commuting and winter use etc) mechanical discs are preferable to this sort of thing, and drum brakes are probably preferable to any sort of disc.

cheers
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Jezrant
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Joined: 14 Dec 2007, 8:11pm

Re: Anatomy of a failed hydro caliper

Post by Jezrant »

Shimano hydraulic discs seem to have this reputation for leaking. Are discs using Dot 5.1 prone to leaky pistons to the same degree?
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of a failed hydro caliper

Post by Brucey »

with DOT 3, DOT4, DOT 5.1 fluids (which are miscible and not related to DOT 5.0 which is different) then corrosion is liable to be worse; these fluids absorb water and if they leak they can even take the paint off the working parts of the brakes.

cheers
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Jezrant
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Re: Anatomy of a failed hydro caliper

Post by Jezrant »

Guess I've been lucky then. I've been running Avids now for seven years. :shock:. Thought it was four or five years but just checked. I've had sticky pistons but that was simple to fix. I've been toying with the idea of swapping the Avids for Shimano hydros because I'd rather use mineral oil than Dot, but been reading about this problem with the leaky piston seals.
Lot of "but"(s) in that post. :lol:
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of a failed hydro caliper

Post by Brucey »

it is fair to say that if you ride offroad, typical mud is a lot less corrosive than winter road salt is. I have had good service from MTB hydros, whereas the same construction of brakes is pretty obviously going to corrode horribly if used on the road.

BTW you can usually tell when AVIDs start to leak; the DOT fluid just strips the paint off the caliper and/or MC.... :wink:

cheers
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Jezrant
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Re: Anatomy of a failed hydro caliper

Post by Jezrant »

You've posted many times useful warnings about the corrosive power of road salt in the winter, and I've seen the sort of damage it can do first hand. I had to replace the wishbones on a twelve-year-old Toyota after they had been turned into Swiss cheese. I was going to add that the Avids are indeed on an MTB that has been used mainly off-road. Haven't noticed any paint stripped, yet. What I'd been wondering about is whether hydros using mineral oil were inherently more reliable than those using DOT 5.1 etc.

There's endless discussion about leaky Shimano hydros on MTB forums. This chap thinks the problem is an O ring rather than the piston seal:
http://blueliquidlabs.com/repairs/solvi ... ke-squeal/
Brucey
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Re: Anatomy of a failed hydro caliper

Post by Brucey »

Jezrant wrote:.... What I'd been wondering about is whether hydros using mineral oil were inherently more reliable than those using DOT 5.1 etc.


I've been wondering the same thing. Some mineral oil calipers seem more prone than others to sticky pistons, even when new, and the first thing to try is to put a drop of oil on the outside of the piston and to work the brake. This must provide some benefit, rather than none, corrosion resistance wise. I also wonder if the cheapskate's oil (Castrol LHM) contains more, better, corrosion inhibitors than the 'correct' mineral oil. If shimano brake oil is anything like shimano grease, it'll be very light on corrosion inhibitors...

There's endless discussion about leaky Shimano hydros on MTB forums. This chap thinks the problem is an O ring rather than the piston seal:
http://blueliquidlabs.com/repairs/solvi ... ke-squeal/


He's probably right; I've seen many reports of this. I wasn't sure that the leakage in the caliper here was at the piston seals or the O ring until I stripped the caliper down. What I did find was that the caliper halves had clearly been assembled with a little mastic or something, so that the facing halves were mostly still bright and shiny. However the O ring had gone hard and was expected to seal in an imperfectly machined recess, so a leak there was only a matter of time I reckon.

BTW one feature of the O ring fitment is non-standard, in that there is just a plain hole through the middle of the O ring; normally the groove completely surrounds the O-ring on all sides so that the static pressure on the seal can be as high as you like: The bulk modulus of rubber is the same as steel; if you make the volume of a surrounding groove smaller than that of the O-ring, the joint won't even close. In this design the O-ring just squishes into the centre bore, and the stiffness of the rubber limits the static seal pressure. If the holes line up well enough (they may not, they are angled) then I may try adding a short length of steel tube through the joint to support the ID of the O-ring, allow higher seal pressure, and hopefully get improved sealing.

I have at least one more caliper to look at, but it has security torx bolts holding it together, and I'm dashed if I can find the right tools (I have them somewhere.... I think...). I'll post more when I've stripped that. I'll also try cleaning up the above caliper to see if the damage can be overcome that way; I suspect that, depending on the type of aluminium alloy and the nature of the corrosion itself, there either will or won't be pitting and this may limit the possibilities for obtaining a seal again in the piston groove.

cheers
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Brucey
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Re: Anatomy of a failed hydro caliper

Post by Brucey »

I just stripped another failed caliper; a BR-M615 model. This had manifested a very slight weep; so slight that contamination from other sources couldn't be ruled out until a couple of sets of new pads had gone the same way, with the brake becoming ineffective and noisy over a period of a few weeks, and an oily sheen developing over the caliper body.

This caliper also had corrosion in the piston seal grooves, but not enough to cause a permanent leak, I'd say. However because the seal flexes and moves in service, it seems likely that the seal would let a very small amount of fluid past every time the brake is used. The corrosion in the seal groove was in the lowest part, where you could imagine water would be most likely to pool.

The joint was protected by mastic as per the other caliper but the O-ring was a long way from being perfect too; sealing faces were less well machined and in addition there were burrs from the centre drillings that marked up the O ring and also could have caused a tiny leak when the O-ring moves in service.

The BR-M615 caliper uses the same materials, seals, and pistons as the BR-M447 caliper; the main functional difference is that the bleed port and transfer port drillings are differently executed, meaning that the BR-M615 caliper would be more likely to fill and bleed using a bottom up push of fluid, but only if the caliper body is positioned at one angle. In particular if the caliper is mounted on the chainstay, I don't think it would fill and bleed reliably unless the angle is changed during the bleed process.

So in conclusion it is a caliper that might have leaked from new (because of a bad joint O-ring arrangement), and that was definitely going to leak worse over time, due to corrosion in the seal grooves, if nothing else.

I'll post photos when I get the chance.

cheers
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Brucey
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Re: Anatomy of a failed hydro caliper

Post by Brucey »

just cleaned up the seal groove in one of the caliper halves from the first post

before
Image

after;
not perfect but better than I expected
not perfect but better than I expected


There is still some damage evident but I think it will seal now.

The method I used was a ~22mm dia flat disc-shaped brush mounted in a dremel tool, run at low speed, with various kinds of polishing compound.

cheers
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NATURAL ANKLING
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Re: Anatomy of a failed hydro caliper

Post by NATURAL ANKLING »

Hi,
On cars and motorcycles the solution to giving long trouble free life is to change the fluid every year.
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Jezrant
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Joined: 14 Dec 2007, 8:11pm

Re: Anatomy of a failed hydro caliper

Post by Jezrant »

That's the solution to lining the pockets of garage owners. Hydraulic brake fluid only needs to be changed if it's contaminated.
Brucey
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Re: Anatomy of a failed hydro caliper

Post by Brucey »

In this case the caliper body has corroded from the outside because it has been exposed to winter road salt. The type of brake fluid used would be almost immaterial. I think that changing it at any interval would have made no difference whatsoever.

Nearly all car brakes use DOT3, 4 or 5.1 fluid, which absorbs moisture very easily. For example if you leave the lid off a container of such brake fluid, it will absorb enough moisture from the air, overnight, that it shouldn't be used. Moisture makes its way into the brake fluid through the reservoir cap and it also permeates through the flexible hoses, and past the caliper seals. The same thing happens on bicycle brakes that use DOT fluid, but actual failures due to moisture absorption are comparatively rare.

You can test brake fluid for moisture content electrically; over 3% and the boiling point of the fluid is depressed enough that you will get boiling if the brakes are used hard. Much more than this and the corrosion inhibitors in the fluid are soon overwhelmed and the system can corrode internally.

In most brake systems with DOT fluid, it should be changed once a year or once every eighteen months if you want to keep it in good order. Failure of an ABS unit due to internal corrosion is liable to be way more expensive than any number of fluid changes.

cheers
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NUKe
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Re: Anatomy of a failed hydro caliper

Post by NUKe »

one of the internet Gurus suggested using Silicon grease, applying to the piston and then pushing fully in when exercising during a brake service. I am replacing a leaking M515 (I think at the weekend), replacing with M6000 this weekend . This a real problem for commuting. I have lost 3 Callipers over the last 2 winters. Avid's didn't fair an better, I had always assumed down to my ham fisted servicing, but Now I am wondering.
NUKe
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NATURAL ANKLING
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Re: Anatomy of a failed hydro caliper

Post by NATURAL ANKLING »

Hi,
Jezrant wrote:That's the solution to lining the pockets of garage owners. Hydraulic brake fluid only needs to be changed if it's contaminated.

Rowlocks!
A litre of fluid and less than one hour is not going to kill the bank.
In my experience on motorcycles and cars, keeping three cars on the road for decades / doing all my own work for mot's for 40 years..........never employed a mechanic.........means that fluid system parts barely give trouble at all.

I will admit that motorcycles and modern bicycles may need better than yearly maintenance for sure.

If you want to be a bit more penny pinching then change brake fluid as per manufactures recommended periods which will be 2-3 years.

Important, some especially older steel car slave cylinders will need changing anyway periodically every five years depending on how used and where you live.

Manufactures recommend changing slave and master seals as a maintenance periodically.
All I am saying is that changing brake fluid (not silicone, I know nothing of this as I never use it) eliminates most of brake fluid problems.

How do you know its contaminated................................?

I keep cars for decades but if you can afford changing car every 3 years for new then you will never get your hands dirty.

Garage the car and do 2000 miles a year and you might never have a problem, put car in garage when its dry, how many of us do that.
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
User avatar
NATURAL ANKLING
Posts: 13780
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: Anatomy of a failed hydro caliper

Post by NATURAL ANKLING »

Hi,
NUKe wrote:one of the internet Gurus suggested using Silicon grease, applying to the piston and then pushing fully in when exercising during a brake service. I am replacing a leaking M515 (I think at the weekend), replacing with M6000 this weekend . This a real problem for commuting. I have lost 3 Callipers over the last 2 winters. Avid's didn't fair an better, I had always assumed down to my ham fisted servicing, but Now I am wondering.


YES, lubing the piston on the slave will also eliminate many problems, just make sure its silicone recommended with what fluid you are using?
Normal non silicon brake fluid, its Castrol red rubber grease, liberally applied.................

Of course cars and m/cycles normally have dust bellow seals as well as fluid seals, I see my old motorcycle last new one I bought has plastic portion on piston beyond the fluid seal air side, which eliminates corrosion and seals without lube......
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
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