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2004 yz125 rear brake doesnt work

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i have a 2004 yz125 and the rear brake was really weak so i rebuilt the master cylinder and it still hasnt got better, do you think its the caliber or something else

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Dirtbike brakes can be tricky to bleed n get the air compleley out of the system. I found the easiest n most foolproof way is to fill the system with brake fluid thru the caliper bleed valve. I do this by using a fork fluid syringe (see pic) , removing the needle on the end of the hose then connecting the hose directly to the caliper bleed valve. Open the valve n force fluid thru the caliper, up thru the brake line n ultimately filling the master cylinder. Works every time n is very easy to do. Could probably go to an auto parts store or maybe even Bed Bath n Beyond to get a syringe then you'd only need a hose. Syringe needs to be a decent size for amount of brake fluid u may need (the one I have works perfect)

U can just keep bleeding it it should eventually begin to work (it can take quite some time n waste good fluid unless u catch it as it's being bled) or could use this method. Screenshot_20190415-221856_Chrome.jpeg

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2 minutes ago, delawhere said:

Dirtbike brakes can be tricky to bleed n get the air compleley out of the system. I found the easiest n most foolproof way is to fill the system with brake fluid thru the caliper bleed valve. I do this by using a fork fluid syringe (see pic) , removing the needle on the end of the hose then connecting the hose directly to the caliper bleed valve. Open the valve n force fluid thru the caliper, up thru the brake line n ultimately filling the master cylinder. Works every time n is very easy to do. Could probably go to an auto parts store or maybe even Bed Bath n Beyond to get a syringe then you'd only need a hose. Syringe needs to be a decent size for amount of brake fluid u may need (the one I have works perfect)

U can just keep bleeding it it should eventually begin to work (it can take quite some time n waste good fluid unless u catch it as it's being bled) or could use this method. Screenshot_20190415-221856_Chrome.jpeg

yea ive tried that but not for very long just till fluid was starting to pop up in the cylinder

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You'll also need to engage the lever as the master cylinder is being filled

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Just now, delawhere said:

You'll also need to engage the lever as the master cylinder is being filled

ok ill try that

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Its been a while but I'm pretty sure I just took a rubber band or something n had the lever engaged during the entire process. Also, as I'm sure u already know, close the valve before u remove the hose. U can just twist the valve with the hose on just to close it then tighten it when u remove the hose.

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1 minute ago, delawhere said:

Its been a while but I'm pretty sure I just took a rubber band or something n had the lever engaged during the entire process. Also, as I'm sure u already know, close the valve before u remove the hose. U can just twist the valve with the hose on just to close it then tighten it when u remove the hose.

on yamahas you can just put a screw driver threw the swingarm bolt and it will hold it down

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on yamahas you can just put a screw driver threw the swingarm bolt and it will hold it down
Yea I have a few, TTR125, YZ250, YZ250F... I keep forgetting we're talking about the rear brake, I keep thinking of front but process is the same.

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When in doubt, rebuild master, caliper, and replace the line. 

Motosport is buy one, get the other free right now on Galfer

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Also make sure the pads are seated properly in the groves/slots they in on the caliper. I had this problem on my wr450 a year or so ago and the bottom of the pad closest to the piston was out of said groove rear brake didn't work. Got the pad situated correctly then the brake worked fine. 

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Rear brakes are a bitch to bleed for some reason.  With a combination of back bleeding from the caliper, bleeding air from the banjo fittings and lots of perseverance they always come around.

Hopefully you used genuine parts to rebuild that master cylinder and not some cheap garbage off of ebay.  The real bummer is that the master probably didn't need to rebuilt in the first place.

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