Author Topic: Brake pads contamination  (Read 882 times)

Ben T

  • What you saying, then?
Brake pads contamination
« on: December 30, 2018, 03:01:54 pm »
My rear brake pads seem to keep getting contaminated. (I say keep - they have done twice now.)

Whilst this is arguably better than the front ones becoming contaminated, it would be quite nice to know why.

Could be over-oiling of the chain and some tiny droplets of chain oil keep flying off and hitting the rotor. I use drip oil rather than spray oil but will try being slightly more rigorous in wiping off the excess, and in such a way as it doesn't get flung near to the brake.

Have left it for a bit in the hope the contamination is only on the surface and will 'wear off', but seems not to be.

Keen to hear opinions on the merits of (if/when contamination does happen) replacing the pads without replacing the rotor (rotors are more expensive than pads), and of replacing the pads with metallic rather than organic ones (I'm wondering if it's harder for contaminants to sink below the surface of metallic pads, and rotors, than into organic pads?) - as well as any preventative measures.
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zigzag

  • unfuckwithable
Re: Brake pads contamination
« Reply #1 on: December 30, 2018, 03:41:25 pm »
there could be several different reasons for the brake pad contamination. road grime sprayed by the front tyre onto the rear rotor, then transferred to the pads. or (in case of hydraulic brakes) a slow leak of oil/brake fluid through the seals. if the chain is lubed with aerosol lubricant near the cassette it could end up on the rotor too, but not the case for drip lubing.
there's no need to change the rotor if it was degreased before the replacement pads were installed.

Re: Brake pads contamination
« Reply #2 on: December 30, 2018, 04:31:33 pm »
What brand are the brakes?
I've had problems with the seals on the cheaper Shimano brakes eventually failing and then leaking onto the pads.
I replaced the calipers and cleaned the rotors thoroughly with IPA and they've been fine ever since.
I think the calipers were damaged by a bike shop - if you use a screwdriver to push the calipers back in, you can scratch the caliper seals.

Re: Brake pads contamination
« Reply #3 on: December 30, 2018, 05:55:32 pm »
I've stripped down half a dozen leaky shimano hydraulic brakes and the cause was the same in each case; corrosion in the seal groove. This is more likely to happen to  rear brake because it sees more road spray, and winter salt water is probably the main cause. There are also reports of shimano calipers leaking at the caliper half joint too.

In such cases the brake mysteriously became contaminated at increasingly short intervals. The brake can be tested for leakage by packing out with tissues, and then leaving the lever pulled to the bar using an elastic band or something, so that the system is under pressure overnight. Any leakage should be quite evident.

It is also possible to get contamination from chain lube or even road film (there is often diesel in road film).

cheers

Re: Brake pads contamination
« Reply #4 on: December 30, 2018, 10:56:02 pm »
Some people swear by a blowlamp for decontaminating pads.
Never tell me the odds.

Kim

  • 2nd in the world
Re: Brake pads contamination
« Reply #5 on: December 30, 2018, 11:10:47 pm »
Some people swear by a blowlamp for decontaminating pads.

I can imagine that working better for sintered than for organic pads...
To ride the Windcheetah, first, you must embrace the cantilever...

Gattopardo

  • Lord of the sith
  • Overseaing the building of the death star
Re: Brake pads contamination
« Reply #6 on: December 30, 2018, 11:17:42 pm »
Some people swear by a blowlamp for decontaminating pads.

Works and is odd to see something bubbling off out of the pads.

Ben T

  • What you saying, then?
Re: Brake pads contamination
« Reply #7 on: December 31, 2018, 10:21:14 am »
I've stripped down half a dozen leaky shimano hydraulic brakes and the cause was the same in each case; corrosion in the seal groove. This is more likely to happen to  rear brake because it sees more road spray, and winter salt water is probably the main cause. There are also reports of shimano calipers leaking at the caliper half joint too.

In such cases the brake mysteriously became contaminated at increasingly short intervals. The brake can be tested for leakage by packing out with tissues, and then leaving the lever pulled to the bar using an elastic band or something, so that the system is under pressure overnight. Any leakage should be quite evident.

It is also possible to get contamination from chain lube or even road film (there is often diesel in road film).

cheers

I would doubt that they're corroded because they're fairly new (this year) and although it's winter now I did put silicon grease on as much of the pistons as I could get to come out of the caliper as best I could.
Also when pushing them back in I generally use a plastic tyre lever.

It's a possibility though so thanks for the useful suggestion of how to test for it.

Tissues with the pads still in? Just thinking the dirt on the pads surface would make the tissues dirty and may obscure any oil traces.
Unless you put on overalls, boots, and a helmet with a high tech pre fitted lamp - and you dig coal - nope, you don't know me.

Re: Brake pads contamination
« Reply #8 on: December 31, 2018, 11:12:32 am »
there are lots of ways of doing a test for leakage, e.g. if you assemble a spacer in situ (eg from a stack of suitably sized coins) so that it is at least as thick as the pad backings plus the disc, you can have this wrapped in clean tissues and clamped by the caliper (elastic band or two around the lever). Any leakage overnight should be obvious.  If a new-ish caliper is leaking, it is likely to be from the seal between the caliper halves.

If you suspect external contamination, take a look at the hubshell and the wheel rim; those will be contaminated too, most likely.

cheers

Re: Brake pads contamination
« Reply #9 on: December 31, 2018, 12:06:52 pm »
To check for leaks, just hold the brake lever on with something elastic (like that yellow Livestrong wristband you will never wear again) and measure the clearance between the lever and the bar.  Leave overnight and then see if it's moved.  This won't show where the leak is but it will confirm if there is one.
Never tell me the odds.

Ben T

  • What you saying, then?
Re: Brake pads contamination
« Reply #10 on: December 31, 2018, 07:14:31 pm »
Oh yeah course the yellow plastic bleeding spacer should suffice. Will give that a try, cheers.
(Hopefully won't be necessary but am quite surprised to learn from investigating it that if it were, replacement of the caliper seems relatively painless, both in cost and in job. Was thinking it might be several hundreds of pounds and the whole hose would need replacing, threading through the frame, PITA connection to the shifter, new olive, new special cutting tools,  yada yada yada, but it seems it's only 40-odd quid and the hose just screws in at the caliper end.)
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Re: Brake pads contamination
« Reply #11 on: December 31, 2018, 09:04:04 pm »
I've gone through multiple (maybe 4 or 5) cheap shimano callipers. I wish they'd make them with piston seals that last.  Its like going back 30 years where a piece of kit can be knackered in a year.

I've never opened a calliper up to see where the leaks are from, but from reading its the piston seals which are the issue. (I've got 2 callipers in the bin which I think I will)

Ben T

  • What you saying, then?
Re: Brake pads contamination
« Reply #12 on: January 01, 2019, 11:59:05 am »
With Shimano calipers, is it possible to take the pistons fully out, in an unsacrificial way?

When you take the disc and pads out, and you pump the lever, the pistons only come out a couple of mm. Certainly no chance of them falling out and oil coming out.

When you say "cheap" Shimano calipers, which sort are you talking about? Do different "grades" have different seal quality I wonder?
The ones I saw online at 40 odd quid are R8070 which are the ones my bike came with.

There was no oil on the tissue covering the spacer overnight and the cable tie holding the lever shut was still at the same tension, so don't think mine is leaking, must just have got unlucky multiple times with either an oily puddle or chain oil spray.
Wonder if one of those plastic discs that is intended to stop the chain going into the spokes might prevent oil coming off the chain hitting it? Fashion faux pas though.  :facepalm: ;)
Unless you put on overalls, boots, and a helmet with a high tech pre fitted lamp - and you dig coal - nope, you don't know me.

Re: Brake pads contamination
« Reply #13 on: January 03, 2019, 10:28:44 pm »
...
When you say "cheap" Shimano calipers, which sort are you talking about? Do different "grades" have different seal quality I wonder?
The ones I saw online at 40 odd quid are R8070 which are the ones my bike came with....

I've gone through shimano deore M6000 and/or M446 : I cannot remember.  One time I've tried a replacement with an XT M785 (or M8000, again I cannot tell difference) to see if XTs last any longer than Deores.  I hope the higher end models have better sealing.

Re: Brake pads contamination
« Reply #14 on: January 03, 2019, 11:28:30 pm »
in the calipers I have taken apart, the seals were fine, nothing wrong with them. The leakage occurred because the caliper body corroded beneath the (largely static) interface to the seal, in the caliper body groove area.


Photos here

https://forum.cyclinguk.org/viewtopic.php?f=5&t=122398

basic shimano calipers use a polished aluminium surface inside the caliper body bore and this is not corrosion proof. I don't know whether posher models use a more corrosion resistant arrangement or not.

cheers

Ben T

  • What you saying, then?
Re: Brake pads contamination
« Reply #15 on: January 04, 2019, 10:41:56 am »
in the calipers I have taken apart, the seals were fine, nothing wrong with them. The leakage occurred because the caliper body corroded beneath the (largely static) interface to the seal, in the caliper body groove area.


Photos here

https://forum.cyclinguk.org/viewtopic.php?f=5&t=122398

basic shimano calipers use a polished aluminium surface inside the caliper body bore and this is not corrosion proof. I don't know whether posher models use a more corrosion resistant arrangement or not.

cheers

That definitely isn't happening in my case.

My point about cheapness was that if even an R8070 (ultegra) one is only 40 quid why bother even contemplating getting an even cheaper one.

Appreciate this may be talking at cross purposes to sg37409 who is on about MTB ones.... but I did once send an entire hope brake back to hope because it needed bleeding and was a bit sticky in returning (but wasn't showing any signs of leaking/contamination) and they sent it back saying they had "replaced corroded caliper" - all this being when it was a few years old and at no charge.
Unless you put on overalls, boots, and a helmet with a high tech pre fitted lamp - and you dig coal - nope, you don't know me.

Re: Brake pads contamination
« Reply #16 on: January 04, 2019, 01:43:31 pm »

….That definitely isn't happening in my case....


Have you carried out the leakdown test as recommended upthread? 

BTW if there is a leak from the piston area you can't see the right part of the caliper to see if it is corroded or not unless you strip it down.  The failed calipers I have stripped down mostly looked OK on the outside, just oilier than they should be.   I am pretty sure it was road salt that was the culprit but caustic cleaners might well cause corrosion too.

BTW some shimano parts get more prone to failures through corrosion, not less, in the more expensive versions.

cheers

Ben T

  • What you saying, then?
Re: Brake pads contamination
« Reply #17 on: January 04, 2019, 03:10:45 pm »

….That definitely isn't happening in my case....


Have you carried out the leakdown test as recommended upthread? 


Yes (last paragraph, Reply #12). There doesn't seem to be a leak.

I had the yellow bleeding spacer in overnight with a tissue round it with the lever pulled shut. There wasn't any oil on the tissue the next morning. A little tiny bit of unavoidable dirt which it gains within seconds from contact, but no actual oil, it was dry.
Unless you put on overalls, boots, and a helmet with a high tech pre fitted lamp - and you dig coal - nope, you don't know me.

Re: Brake pads contamination
« Reply #18 on: January 07, 2019, 11:23:31 pm »
I took one of the calipers apart tonight, and I think the leak is from around one of the piston.
Your photos are interesting : How did you get the pistons out ? I could not extract them.
Both pistons now seem to be bottomed out, when I press on one of them I can just see some fluid showing around a third of the circumference of the piston.

The transfer port looked fine; clean and corrosion free and no discolouration around it.
thanks
Steve

Re: Brake pads contamination
« Reply #19 on: January 08, 2019, 12:54:23 am »
it is best if you decide you are going to remove the pistons before you break the hydraulic circuit; just pump the pistons out using the brake lever, making sure that each piston doesn't come out too far; as soon as one piston comes out (completely past the seal) the other one will no longer move.

To keep the pistons from coming out too far you can put a spacer in the caliper, such that it also is a tight fit in the caliper body slot, and will stop one piston from going too far if needs be.  You are always 'safe' if a centrally positioned spacer is no thinner than the minimum thickness allowable for a disc plus twice the pad backing thickness. This is normally about 4.5mm but it can vary with the brake design.

With most run-of-the-mill shimano brakes (with polymer pistons) the seal is about 2.5mm recessed in the caliper body. This means that scratches near the every end of the piston won't cause a leak, so if you have to you can grip the piston near the end even if it marks it slightly. However once the pistons are pumped out as per above you usually don't need to struggle to remove them entirely.

With calipers that are no longer connected to the hydraulics (or caliper halves with pistons in) you can use a track pump or compressed air to remove the pistons; be careful though; they will easily become projectiles if you use compressed air this way. If you have one of those pointy plastic nozzles that are for inflating airbeds, it is usually possible to seal it well enough to get pistons out.

 If they are really stuck (and BTW this works really well for car brakes too) you can use a decent grease gun to pump the pistons out. Grease guns develop thousands of psi and I have not yet failed using this technique.  This way the pistons won't become projectiles either. However there is one snag; grease can soon attack the seals, so if you want to have a chance to re-use them it is very important to remove and clean the seals immediately, ideally wiping the seals down with clean brake fluid of the correct type to make sure they are completely free of grease.

cheers

Re: Brake pads contamination
« Reply #20 on: January 08, 2019, 05:38:03 pm »
Thanks Bruce - I guessed I should have pushed the pistons out before I cracked the circuit.  I’ve got another in the scrap pile which I’ll have a go on
Steve

Ben T

  • What you saying, then?
Re: Brake pads contamination
« Reply #21 on: January 08, 2019, 08:33:46 pm »
Lots of good ideas on avoiding contamination from the brake fluid through the piston seals, but any theories on whether sintered/metal pads are any better at resisting contamination from elsewhere than organic pads?

I have a theory (maybe just a hope) that oil could be more likely to soak into organic pads whereas with sintered/metal ones it may just contaminate the surface and be wiped off either by the disc during riding or sandpaper/cloth at home. Any doubts on this?
Unless you put on overalls, boots, and a helmet with a high tech pre fitted lamp - and you dig coal - nope, you don't know me.

Re: Brake pads contamination
« Reply #22 on: January 08, 2019, 08:58:13 pm »
sintered pads are still porous but a metal-on-metal contact may well be inherently more resistant to contamination; IIRC the compound usually contains some lower melting point metal components that act as a 'lubricant', else you would weld the pads to the disc.  Even with a little oil present, they still have more friction than a similarly contaminated organic pad.

IMHO the big advantage that sintered pads have  is that if they are badly contaminated with oil, you can usually clean them; you can pretty much set light to the whole pad so that all the oil burns off.

cheers