Bleedin’ Hydraulic

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belgiangoth
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Bleedin’ Hydraulic

Postby belgiangoth » 9 Feb 2020, 8:48pm

Rear brake was a bit squishy, so got the the brake fluid done at the shop. It has been months, but not many rides and the rear brake is gone again. Front is fine. Any thoughts what might have gone wrong?
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DevonDamo
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Re: Bleedin’ Hydraulic

Postby DevonDamo » 9 Feb 2020, 10:20pm

belgiangoth wrote:Rear brake was a bit squishy, so got the the brake fluid done at the shop. It has been months, but not many rides and the rear brake is gone again. Front is fine. Any thoughts what might have gone wrong?


There are two situations I've encountered which have led to my rear brakes fading suddenly. Firstly, if you contaminate your disc with grease, then it will feel as though the system has lost all hydraulic pressure, but this is just an illusion - the pads are still squeezing against the disc, but there's no friction. Secondly, if you turn your bike upside down and operate the brake lever, you can get air in the system.

In both cases, I've been able to sort it out by riding. A long downhill section with the brake held on full will usually burn off any contaminant from the disc. And any air in the system will usually work its way back out after you've been pumping the brakes for a few minutes.

peetee
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Re: Bleedin’ Hydraulic

Postby peetee » 9 Feb 2020, 10:40pm

What is the make and model of brake and how and where is the bike stored?
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philvantwo
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Re: Bleedin’ Hydraulic

Postby philvantwo » 9 Feb 2020, 10:49pm

How can the air work itself out?
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hemo
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Re: Bleedin’ Hydraulic

Postby hemo » 9 Feb 2020, 11:41pm

If the reservoir has any air space then upturning causes it to travel along the brake line to the highest point (calliper), the air tends to stay in the line and not enter the calliper. Inverting bike again it should eventually rise back up to the reservoir if the lever is pumped enough, gently tapping the line along it's length also helps along with pumping the brakes. Back at home if not cured leave over night with lever fully depressed with some lacky bands or Velcro, the air should travel back to the top.

Brucey
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Re: Bleedin’ Hydraulic

Postby Brucey » 10 Feb 2020, 1:37pm

yep that is about it. It should be possible to bleed the systems such that there is no air in it and there is the correct amount of fluid, but this is commonly not achieved.

FWIW getting the correct amount of fluid in the brake reservoir, 100% air-free, is non-trivial. If it is done right there should be enough fluid so that the brake works OK until the pads are worn down, but if there is not enough fluid in the system, the brake can go all mushy when the pads are part worn, whether there is air in the system or not.

There are also many other types of hydraulic fault that can also stop the brake from working, but these usually involve fairly obvious symptoms like fluid coming out, seized pistons and so forth.

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Gattonero
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Re: Bleedin’ Hydraulic

Postby Gattonero » 18 Feb 2020, 2:22pm

belgiangoth wrote:Rear brake was a bit squishy, so got the the brake fluid done at the shop. It has been months, but not many rides and the rear brake is gone again. Front is fine. Any thoughts what might have gone wrong?


Unfortunately that is a bit generic, so cannot give much help....
The word "squishy" if very personal, from what I have seen there is the general consensus that disk brakes have to "read your mind and stop on a dime with 2mm of lever travel". I think you are experienced enough to not believe this, so we are talking of a brake lever that almost goes to the bars?
Does your bike have internal cable routing, running under the BB? This could be a start...
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PH
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Re: Bleedin’ Hydraulic

Postby PH » 18 Feb 2020, 6:04pm

I'm a newcomer to hydraulic brakes (Shimano) and have found the instructions from Epic to be clear, simple to follow and effective. Their kits and parts are also competitive, though the information applies to any kit.
https://epicbleedsolutions.com/pages/resources

It seemed a bit daunting compared to cable brakes, rim and disc, though the reality is it's pretty simple stuff, just follow the instructions.