SLX M675 - Air in system
JodyP
Posts: 193
Hopefully someone can help with this. I swapped brakes to a pair of SLX a few months ago but have problems with what I think is air ingress at the levers. When going down fast bumpy descents the lever travel would pull back almost to the bar, but when pumped would come back. It would also sometimes do the same if turning the handlebars 90 degrees whilst on the spot. Sometimes they would come back straight away other times would be 10 minutes or so. Ordered a bleed kit and did both. Air came out of both lines and all seemed well. However had to let go of the bike or face going OTB on Sunday and the rear started playing up. Lever didn’t take a knock but the bars span 90 degrees.
The only thing I can think is air is getting past the olive and into the master cylinder but the line is nipped up tight.
Any ideas?
The only thing I can think is air is getting past the olive and into the master cylinder but the line is nipped up tight.
Any ideas?
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Comments
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Sounds like air still in the system or a passing seal. I would rebleed it and try again, I always bleed it then let it stand a bit then rebleed them seems to work for me. If not fully sure on how to do it follow one of the many youtube clips.
No issues following the bleed instructions but will have to try again to rule it out.0 -
Sounds more like a pad knock off issue than bleeding.Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.0
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Sounds more like a pad knock off issue than bleeding.
What is pad knock off?0 -
Where the pad gets 'knocked off' the disc (usually by distribution in the fork lowers, axle inc. QR, hub or disc) so that your initial level pull is just closing the gap down between pad and disc and the next one actually pushing the pads onto the disc.Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.0
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Where the pad gets 'knocked off' the disc (usually by distribution in the fork lowers, axle inc. QR, hub or disc) so that your initial level pull is just closing the gap down between pad and disc and the next one actually pushing the pads onto the disc.
It would make sense on the initial pull and then coming back with a couple of pumps. But doesnt explain the 10 minute loss of brakes or when I lose back brake when turning the handlebars quickly on the spot. Its certainly not a problem with pistons needing to be advanced.0 -
Turning the bars could knock the pads off.....Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.0
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Turning the bars could knock the pads off.....
On the rear caliper?0 -
If the bike does anything put stay stationary, yes, for example if the rear QR is loose then the wheel can wobble (anything that could cause the rear wheel to wobble relative to the calliper, so even loose FS pivots) as you move the bars and knock the pads off.Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.0
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Hmmm, I don't know about knock off.
The only time I have experienced knock off was over corrugated corners rallying.
I can't imagine any bicycle disc would have enough stiffness to push the pads back to achieve this.
A motor vehicle disc is much much stiffer and it usually takes extremely high loads to achieve this and usually there is found to be loose wheel bearings or similar.
I just cannot imagine a flexi bike disc achieving this. I certainly never experienced it even running totally trashed lower rear arm FSR bushings.
I'd love to hear from others if they have experienced a confirmed case of this.
My bet is that he has used the wrong sized inserts in the line when he shortened them.
Many people don't realise that there are silver, bigger diameter, and gold, smaller diameter ones.
Although I always assumed that was an internal measurement so it still should work and if he bought a whole kit it should have come with the correct ones anyways.
Really just sounds like a bad bleed to me, but seriously, late model Shimano pretty much bleed themselves, it's nigh on impossible to stuff it up.
Keen to see what the answer ends up being.0 -
Could it be a loose hose connection? With movement it is letting a bit of air into the system. Incorrectly sized inserts mentioned above would have a similar effect.0
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Thanks for the reply Rightarmbad and Veronese. I am swaying towards the inserts or olives allowing a little air past into the master cylinder. Both brakes were bought new as a kit and came with the inserts, olives etc. Its news to me that there are different size hose inserts. As you say the bleed procedure is really straight forward (although not as easy as a car). Wheel bearing, pivot bearing etc are all spot on and giving no knocks or flex in the chassis. I cannot say the same for flex in the Fox Evo's but that's for another thread, lol. The front hasn't had any problems since rebleed and I think the problems were low fluid in the M.C after shortening the hoses.
The only thing that goes against a dodgy connection is the brakes hold pressure and have no visible fluid leaks. I might have a nosy tomorrow night as I can't get out on the bike this weekend.
Thanks for the advise peeps and I will let you know how I go.0 -
You don't hang your bike from the rear wheel for storage do you?
I ask this, as if there is any air in the levers and you hang it that way, the air can migrate to the calipers.
After you opened the lines did you fit the bleed cup with a little fluid and pump the bubbles out?
Air is less viscous than fluid so if you do have poor connections, air may enter without showing a fluid leak.
After a lot of riding there may also show up a little wetter dust around the leak.
Also check pistons for cracks, there have been other reports of this.
Things I would personally do if presented with this in a workshop situation.
(Considering that the only thing done to this system that used to work fine is to play with the connections at the lever.)
Remove caliper, line and lever from bike in one piece. Much quicker overall than tapping lines trying to get bubbles to follow curves and leaves no high spots for bubbles to get trapped.
Mount lever on old bar in a vice and let caliper hang with no place in lines to trap air.
Mark line where it enters lever rubber nut cover to check length of line for full insertion.
Remove line taking note of how tight the nut was. Especially if loose.
Check old line for crimping of line due to too tight fitting, causing possible distortion and leaking. Check if correct insert used.
Cut of old olive and insert and insert line into lever to check total insertion length and compare to original.
If problem has not yet been identified, re-terminate correctly with new insert and olive, fit line, fit cup with a small amount of fluid, remove pads and push out pistons enough to clean sides and inspect for cracks, push all the way back in, squeeze lever and check that both pistons move freely, push back in and fit bleed block.
By this time the system should have been sitting like this for a little while so should only need squeezing and releasing of the lever a few times until bubbles stop appearing in the cup as any air in the system should have already risen to the top aided by the pistons being pushed all the way out for fitting of the bleed block.
Leave system like this and go do another job, occasionally coming back and giving the lever a hard squeeze and release to help bubbles to form as pressure drop when the lever is released.
Come back when finished other job and do the lever squeezing thing again and if no additional bubbles appear, close the system and refit to bike.0
