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Front brake caliper piston?

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So i'm helping a buddy try and get his front brake fixed on his 04' YZ 250F. We installed a master cylinder rebuild kit and still the lever pulls all the way to the bar with no pressure at all. We have correctly bled the conventional way and also back bled pushing fluid up from the bleed port with no luck. As we pump the lever I can see the pistons moving out but they are not moving together and when they make contact they just pulse back and forth, no pressure building. Could this possibly be the piston seals? I would think there would be fluid loss if this is the case, but there is none. No noticable damage or bulging of the brake line either. I'm out of ideas, any help would be appreciated.

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Sounds like the master cylinder needs rebiult, A way to check would be to put a pair of needle nose vise grips on the line right next to the cylinder with just enough pressure to close the line. If you lever is still soft you need a rebiuld and if it gets hard your problem is in the line or caliper.

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He did just rebuild the master.

The yamaha brakes are a pain to bleed and get all the air out. I had to buy a vacuum bleeder to get all the air out of mine. My best guess would be that when you installed the rebuild kit one of the seals did not get in correctly, either it is in backwards or the seal is lipped up a bit. Check that out and then move on. Also i would bench bleed the master before you hook the line back up.

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Bench bleed? Is that just filling the master up with fluid before installing the brake line? I didn't even take off the brake line when I installed the rebuild kit. As far as the seals go, the picture in the manual sucks but think I have it right. Do you think the two pistons comming out at differnet times is an issue or think it is just a product of trapped air? I've bled brakes before and was able to at least get a little spongy feel after awhile but no dice with this one. Thanks again

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when you pull the lever does it grab for a second and then let go? my 03 did this after the master cylinder was rebuilt. i just tossed the rebuilt cylinder and put on a new one. i don't think that this is your problem but this is what mine did so I hope it helps!

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I cant get any pressure or grab at all. Does anybody know if both seals on the master cylinder piston angle or taper inward. Like I said before the pics in the manual are awful. They were put in the same way they came off but the brakes were sketchy when the bike was purchased. Gonna try to raise the caliper above the master cylinder and see if we get anything to bleed out.

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I cant get any pressure or grab at all. Does anybody know if both seals on the master cylinder piston angle or taper inward. Like I said before the pics in the manual are awful. They were put in the same way they came off but the brakes were sketchy when the bike was purchased. Gonna try to raise the caliper above the master cylinder and see if we get anything to bleed out.

i just rebuilt my master cylinder and the "orings" are actually cupped. the open part of the cup should be facing inward toward the spring. they are also designed a little differently and the one with the groove looking things should be the closest to the spring. i feel like this is where your problem is. once i rebuilt mine it works better than ever.

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Yup, got it! One of the little orings on the plunger in the master cylinder was installed backwards at the shop! Thanks for all the replys!!

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Yup, got it! One of the little orings on the plunger in the master cylinder was installed backwards at the shop! Thanks for all the replys!!

Great to here. We all have rebuild something the wrong way before. I am glad we could help.

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