Shimano Xt Brakes Pump Up.

Postby peanutaxis on Sun 5/Jun/16 8:29pm

Hi,


I have this problem with my Shimano XT brakes. They pump up. If I brake, then brake again within about a second, the bite point comes progressively away from the lever. The rears are worse than the fronts. When I do it off the bike while looking at the rear pistons it almost looks like the pistons don't have time to retract back to their resting position, so that if I brake again in quick succession I'm shoving more and more fluid down the pipes before the pistons get to return to their resting position. I wouldn't complain about this problem but it is quite noticeable while riding and a bit disconcerting as the bite point keeps changing!

They were factory bled up until this point (about a year) but then I started noticing this problem with the rear. So I bled them a few days ago but the problem still remains. Thought someone might have some insight.



thanks
p
peanutaxis
Member for: 1 year 5 months

Re: Shimano Xt Brakes Pump Up.

Postby Velocipedestrian on Sun 5/Jun/16 8:47pm

They need a bleed.
Velocipedestrian wrote:You're after the bike mechanics thread.
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Re: Shimano Xt Brakes Pump Up.

Postby AgrAde on Sun 5/Jun/16 8:52pm

They do that when your pads are low or your rotors are thin.
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Re: Shimano Xt Brakes Pump Up.

Postby peanutaxis on Sun 5/Jun/16 9:13pm

Velocipedestrian wrote:They need a bleed.
Velocipedestrian wrote:You're after the bike mechanics thread.


Nope. They have rock solid feel.

And this is the Bicycle Repair section!
peanutaxis
Member for: 1 year 5 months

Re: Shimano Xt Brakes Pump Up.

Postby peanutaxis on Sun 5/Jun/16 9:22pm

AgrAde wrote:They do that when your pads are low or your rotors are thin.


Hmmn. The pads are new in that they have only just 'mated' with the discs (if you know what I mean. The discs are slightly dished as the cleansweeps wear in the middle a bit faster, so the pads mate/abrase on the outside first). Maybe I need to force the pistons out further?
peanutaxis
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Re: Shimano Xt Brakes Pump Up.

Postby Wobbler on Sun 5/Jun/16 9:52pm

Have you lubed the pistons up? The pads springs in good condition?

I recall older shimanos would do that when a lever seal went.

Is the bleed actually good?
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Re: Shimano Xt Brakes Pump Up.

Postby peanutaxis on Sun 5/Jun/16 10:30pm

Wobbler wrote:Have you lubed the pistons up? The pads springs in good condition?

I recall older shimanos would do that when a lever seal went.

Is the bleed actually good?


How do you lube the pistons up? I imagine you have to remove them to do this? Don't they get lubed from the internal mineral oil?

Pad springs are fine, though I believe that what retracts the pistons is the 'square cut seal' (google) - that is the 'spring' in the system. The pad springs just keep the pads pushed against the pistons and away from the disc.

The bite feels as rock solid as when I bought them so I think the bleed is good.

What is the lever seal?
peanutaxis
Member for: 1 year 5 months

Re: Shimano Xt Brakes Pump Up.

Postby BrokenKonaRider on Sun 5/Jun/16 11:00pm

Known problem with the m8000s apparently.I have a similar problem with mine. Its the randomness that annoys me. Half the time its got great modulation, the rest its like the the lever only has to move a cm and I'm skidding. Search MTBR forums.
If youre not using m8000s, take the brake off and bleed it completely vertical, tapping the calliper to get and trapped bubbles out.
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Re: Shimano Xt Brakes Pump Up.

Postby Wobbler on Mon 6/Jun/16 12:58pm

peanutaxis wrote:
Wobbler wrote:Have you lubed the pistons up? The pads springs in good condition?

I recall older shimanos would do that when a lever seal went.

Is the bleed actually good?


How do you lube the pistons up? I imagine you have to remove them to do this? Don't they get lubed from the internal mineral oil?

Pad springs are fine, though I believe that what retracts the pistons is the 'square cut seal' (google) - that is the 'spring' in the system. The pad springs just keep the pads pushed against the pistons and away from the disc.

The bite feels as rock solid as when I bought them so I think the bleed is good.

What is the lever seal?


No you dont take them out to lube them, you just push them out a little way via the lever then run some mineral oil around the edges.

Were there any flecks of any material in the bleeded out fluid?

I'm still thinking you have missed a smidge of air in the caliper that is moving around once you start pumping
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Re: Shimano Xt Brakes Pump Up.

Postby Dougal on Mon 6/Jun/16 1:56pm

You have too much oil in the reservoir. Push the pistons back in and then bleed them.
Dougal
Member for: 14 years 6 months

Re: Shimano Xt Brakes Pump Up.

Postby peanutaxis on Wed 15/Jun/16 3:38pm

Dougal wrote:You have too much oil in the reservoir. Push the pistons back in and then bleed them.


Just tried this today, but it didn't help. The pistons were almost flush already (the pads are quite new). Nomatter what I did the pistons sit just proud of the caliper - I think this is normal - and the bleed block had space to rattle round.

Next I think I'll try lubing the pistons.

When I let the brake lever go and watch the pistons, they really take a long time to return to their set/off position ??
peanutaxis
Member for: 1 year 5 months

Re: Shimano Xt Brakes Pump Up.

Postby Dougal on Wed 15/Jun/16 4:48pm

peanutaxis wrote:Next I think I'll try lubing the pistons.

When I let the brake lever go and watch the pistons, they really take a long time to return to their set/off position ??


Lubing up is a good plan.
Dougal
Member for: 14 years 6 months

Re: Shimano Xt Brakes Pump Up.

Postby shmoodiver on Wed 15/Jun/16 6:04pm

lubing the piston will have the opposite affect to what you're after. Caliper seals are square (cross section) so they deform on closing and revert on fluid release to return the piston, lubing it will cause the piston to slide on the seal and not get the deformation required to return the piston.
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Re: Shimano Xt Brakes Pump Up.

Postby Conners on Wed 15/Jun/16 7:09pm

shmoodiver wrote:lubing the piston will have the opposite affect to what you're after. Caliper seals are square (cross section) so they deform on closing and revert on fluid release to return the piston, lubing it will cause the piston to slide on the seal and not get the deformation required to return the piston.

I'm wondering if the seals are past their use by date - just not giving enough 'spring' anymore?

Disclaimer - I'm no bike mechanic.
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Re: Shimano Xt Brakes Pump Up.

Postby pokey on Wed 15/Jun/16 9:25pm

shmoodiver wrote:lubing the piston will have the opposite affect to what you're after. Caliper seals are square (cross section) so they deform on closing and revert on fluid release to return the piston, lubing it will cause the piston to slide on the seal and not get the deformation required to return the piston.


The spring pushing master cylinder piston back creates the negative pressure to pull the calliper pistons back. The calliper piston seals have all the lube they need on the inside. They need to be clean outside, Shimano recommend isopropyl alcohol or soapy water, not oil for dirt to stick to or bake onto the piston.
It sounds like the OP still has air in the system, https://www.youtube.com/watch?v=9ZYmW8i8TWQ
pokey
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