Bleedin’ Hydraulic

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belgiangoth
Posts: 1268
Joined: 29 Mar 2007, 4:10pm

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
Posts: 272
Joined: 24 May 2011, 1:42am

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
Posts: 1648
Joined: 4 May 2010, 10:20pm
Location: Cornwall

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
Posts: 673
Joined: 8 Dec 2012, 6:08pm

Re: Bleedin’ Hydraulic

Postby philvantwo » 9 Feb 2020, 10:49pm

How can the air work itself out?
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hemo
Posts: 616
Joined: 16 Nov 2017, 5:40pm
Location: West Sussex

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
Posts: 36646
Joined: 4 Jan 2012, 6:25pm

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.

cheers
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