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matgriff

bleeding brakes

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Hi. A few weeks ago I rebuilt my front brake caliper, new pistons and seals (2009 290), had a right job bleeding it but got there in the end by sucking a vacuum on the bleed nipple and pumping it through. last week I lost the little spring off the front brake lever and noticed that the rubber dust cap was missing off the master cylinder and it was a bit corroded around the circlip, so I ordered a new rebuild kit for the master cylinder. I cleaned everything up and fitted it without any problems, Trouble is now I can't get any pressure at the lever, I've tried sucking out through the bleed nipple and topping up the master cylinder, but the lever just keeps pumping with no resistance at all, like it's air locked somehow ??

I've messed about with it for a hour or so with no luck.

 

Any ideas ?

 

Thanks

 

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Normally when I bleed both the front brake & clutch I just jiggle the lever & let the air exit back to the master cylinder, takes time but you'd be surprised at how effective it is. Remember that the air wants to rise to the highest point

It's not possible that you folded part of the seal over when fitting the piston?

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Hi, thanks for the info, yes I tried to get all of the air out of the m/c by jiggling the lever.  I was also thinking that the seal may have folded over, I'll take it apart again later.

 

Mat

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Different things work for me at different times, I've had success with regular bleeding, bottom-up bleeding (with a syringe pumping the fluid from the bottom) and unmounting the caliper and raising it above the master cylinder.

Last Monday I had to redo the front on my Sherco because there were some gooey bits in the reservoir and blockage from the reservoir to the master cylinder.  It decided it didn't want to bleed so after much swearing with all of the above I put the whole system upright, taped to my workbench, with the master cylinder removed but the reservoir top on and poured fluid into the hole for the master cylinder using the bleed nipple to let out a bit of air at a time.  A bit of a sledgehammer to crack a nut, but it's fine now ... I did get brake fluid everywhere though 😅

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I finally fixed it this afternoon.  I think my first problem was that I didn't have any free play at the lever, so the m/c piston wasn't relaxing to the full end stroke. I read this tip on the Beta or Gas Gas forum here, same AJP brakes.  When I backed off the adjuster fully, I was able to just pump the lever slightly back and forth and a lot of small air bubbles started to rise in the m/c. After about 20 minutes I started to get a nice firm lever with no more bubbles. A final "normal" bleed through the caliper nipple with the lever pumped and held seems to have worked.

 

Thanks for all the help.

 

Mat

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2 things that have worked for me in the past.

Firstly I once made the mistake of rebuilding the master cylinder dry, the dam thing wouldn't bleed no matter what I did. I stripped it back down and added some fluid to each part. This seemed to have done the trick.

Secondly if I have had trouble with stubborn brakes or clutches in the past, I've tie strapped the level back to the bars (engaging the piston) topped up the fluid leaving the master cylinder top slight ajar and left it over night. Next day after a couple of normal bleeds I finally get a firm brake/clutch. This has worked every time I have done it so far. 

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On 5/3/2021 at 7:56 AM, Intotrials said:

2 things that have worked for me in the past.

Firstly I once made the mistake of rebuilding the master cylinder dry, the dam thing wouldn't bleed no matter what I did. I stripped it back down and added some fluid to each part. This seemed to have done the trick.

Secondly if I have had trouble with stubborn brakes or clutches in the past, I've tie strapped the level back to the bars (engaging the piston) topped up the fluid leaving the master cylinder top slight ajar and left it over night. Next day after a couple of normal bleeds I finally get a firm brake/clutch. This has worked every time I have done it so far. 

I've always managed to get the job done over the years by sucking a vacuum on the caliper bleed nipple and also as you say tie wrapping the lever back , but for some reason this one has been particularly stubborn. Hopefully it should be good for a while now.

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Just to add to what the others have said, I have recently found that removing the piston all together and filling it slowly/jostling the brake hose to really help. Then just put the piston/clip back in and work the lever slowly for a bit. It helps if you are able to hold the piston(s) completely pressed in with a bit of wood so any oil transfer doesn’t move the pistons. Worth a shot

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7 hours ago, jackman290 said:

Just to add to what the others have said, I have recently found that removing the piston all together and filling it slowly/jostling the brake hose to really help. Then just put the piston/clip back in and work the lever slowly for a bit. It helps if you are able to hold the piston(s) completely pressed in with a bit of wood so any oil transfer doesn’t move the pistons. Worth a shot

I'm glad I'm not the only ones to have done this 😅

A piece of metal/plastic/wood the same thickness as a brake disk is essential to put in the caliper though, yeah.  I have a specific bit of scrap metal that does the job :)

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