SRAM hydro brake issue; suspected contamination.

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Brucey
Posts: 40650
Joined: 4 Jan 2012, 6:25pm

SRAM hydro brake issue; suspected contamination.

Postby Brucey » 15 Sep 2020, 10:44am

recently I helped repair a newish MTB fitted with SRAM hydro brakes (using DOT fluid).

The symptoms were that the brake would just 'stick on' and the lever wouldn't return fully. A little swapping around the MCs showed that the affected caliper was fine ( the brake retracted normally when connected to the other MC) so the finger of suspicion was pointed at the MC, which did indeed seem super sticky when it wasn't connected to anything.

In such cases you have to be suspicious that the wrong fluid has been used in the brakes (even just for top up) and that this may have caused the seals etc to swell.

However what came out of the brake was obviously DOT fluid, or at least mostly DOT fluid, and the MC seals at first sight looked fine. Further investigation involved removing the seals from the MC piston and checking some more. The culprit turned out to be the MC piston itself. This appears to be made in Acetal type material, and one end of the piston (the 'air' end) slid easily into the MC bore but the other end (the 'fluid' end) didn't any more. Careful checking with digital verniers revealed that the MC piston was about 0.1mm oversize at the 'fluid' end which was enough to cause binding, severe enough for the return spring not to cope.

The piston was soon mounted for rotary dressing and a few minutes later it was once again the right size, and functioned correctly. This saved the cost of a rebuild kit at least, and had one not been readily available, a new MC (~£240 was the quote.... :shock: ) and/or a delay. As it was the bike was back on the road (well, trail) the same day and the repair cost was a fair amount less than it might have been.

So there's yet another thing to look out for there.... :roll:

I have questions though;

1) has anyone else seen something similar?

2) if DOT fluid is contaminated with (say) water this can be tested for ( I have a moisture testing meter for exactly this purpose). But is there a means of testing for mineral oil contamination of DOT fluid?

cheers
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tim-b
Posts: 1555
Joined: 10 Oct 2009, 8:20am

Re: SRAM hydro brake issue; suspected contamination.

Postby tim-b » 15 Sep 2020, 11:35am

Hi
Yes, some SRAM Guide models suffered with this and a new piston is available. Anecdotally worse when it's warmer weather
Regards
tim-b
EDIT: Example link
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Brucey
Posts: 40650
Joined: 4 Jan 2012, 6:25pm

Re: SRAM hydro brake issue; suspected contamination.

Postby Brucey » 15 Sep 2020, 11:47am

thanks, that is interesting and relevant; the MC pistons even look the same in the link as the duff one I was working on.

What is a bit weird (and fits with the contamination hypothesis quite well) is that the left and right MCs were the same age, had (presumably) been filled with the same fluid originally, and had seen similar use. Yet one had a sticky piston and the other one didn't; very odd.

BTW the problem was almost certainly swelling of parts in contact with the fluid; the two ends of the duff MC piston were a different size accordingly. That the problem is worse with heat is understandable; the CTE of the plastic piston is higher than that of the (metal) MC bore.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~