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02YZ426

Air In Brake Line 2007 YZ450

23 posts in this topic

The brakes were working perfect, parked the bike for a few months. Then when I was going over the bike to get ready for the indoor track I noticed there was no rear pedal height at all. So I bled the line out and got all kinds of air out. It did come back to proper height and worked perfect all night. Now after sitting another month, its back to no pedal height. Its not leaking oil anywhere. What is going on here? How do I fix it? Thanks.

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Could it be the rubber brake line seaping from the lack of use? Should I change the line to a braided stainless line?

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It has to be leaking somewhere. Check under the boot where the operating rod runs into the bottom of the M/C.

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Well thats just it, no visual signs of leakage anywhere. The fluid level hasn't been dropping either, its always full. Somehow air is getting in though :confused:

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If there is no signs of external leaks. Could be m/c or caliper seals letting air into system. when you apply the brakes and release them there is a low vacumm on the brake system which helps the caliper piston return (also caliper piston square cut seal helps return it to) if the seal in the caliper or in the m/c is weak it may be allowing air in.

also when bleeding air make sure not to pump brake pedal. this can also get air into system.

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You've either got a blown seal, a cracked line (somewhere), OR, and don't take this personally, you didn't bleed the brakes right the first time. The first time I bled my brakes, I had a hell of a time (don't know why :confused: ) getting all the air out. If there's no leakage anywhere, that's where I think your problem would lie.

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when you apply the brakes and release them there is a low vacumm on the brake system which helps the caliper piston return
None of this is true.

The caliper piston does not return anywhere, in the first place. It is advanced only marginally, and if it retrearts from the rortor at all, it is only due to the runout of the rotor, and the play in the wheel bearings. Having it stand more than a very few thousandths away from the rotor creates excessive pedal/lever travel when the brakes are applied, and the only thing necessary to affect a complete release of the brake is for the pressure to be let off the pads.

There should also never be a vacuum in the system either, and the cap on either master cylinder is vented to prevent it. On the front, this can be seen as a tiny notch on the back edge of the reservoir cover. Air it vented to the gap between the cap and the weather seal boot, which expands to allow fluid to migrate into the hydraulic circuit as the pads wear. Caliper seals on most brakes will simply not seal air out when exposed to any kind of vacuum, as anyone with experience with the brakes on C2 and C3 Corvettes knows all too well.

When you release the brake, the lips of the master cylinder piston seal(s) fold down, allowing fluid to flow down over them from behind (where the reservoir is), so they don't apply any vacuum that way, either.

Now, however, the ventilation issue could be what is causing this problem. check the reservoir cap and weather seal to be sure the vent groove is not blocked, or the seal swollen and plugging it. Air entering the system this way will be concentrated near its entry point, so if you get air immediately from the caliper bleeder, it's getting in there. If, OTOH, you can get air to exit more quickly by bleeding at the line fitting on the M/C, the M/C shaft is leaking.

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If there is no signs of external leaks. Could be m/c or caliper seals letting air into system. when you apply the brakes and release them there is a low vacumm on the brake system which helps the caliper piston return (also caliper piston square cut seal helps return it to) if the seal in the caliper or in the m/c is weak it may be allowing air in.

also when bleeding air make sure not to pump brake pedal. this can also get air into system.

Yeah I bet its coming in the caliper, hard to know for sure

You've either got a blown seal, a cracked line (somewhere), OR, and don't take this personally, you didn't bleed the brakes right the first time. The first time I bled my brakes, I had a hell of a time (don't know why :confused: ) getting all the air out. If there's no leakage anywhere, that's where I think your problem would lie.

I'm 100% certain I had all air bled out of the line.. its getting back in though.. not a little either.. . TONS!

I'm going to buy a new brake system off ebay or something. I'll replace each part 1 by 1 until I figure out where the problem is.

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How much oil is supposed to go in the rear master cylinder? Fill it to the top of the sight glass, or fill it to the line inside the cylinder looking from the top?

I just bled the line out tonight again. Pedal height is back to A1. The air is definitely in at the master though. I had to bleed the line almost all the way out before I got the air. I suppose it could still be a bad seal at the caliper and eventually work its way up the line but who knows.

These kind of problems really get to me.. almost a new bike, no reason for it to happen now, and its a pain to pinpoint what even needs repaired!

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On the rear, fill to the line.

To the line on the site glass? or the line inside looking in from the top? There are 2 lines.

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The higher one inside. Look at the weather seal (cap gasket, if you will). It's rather like a bellows so that it can expand into the reservoir as the fluid level drops. What I do is collapse this bellows seal, then fill it to the level where no more fluid will fit once the seal is inserted. That also means that I'll probably need to let some out at pad replacement time, but that's OK.

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Ya thats what I thought. I'm now sure my gasket thing will retract on this one though, I'll have to look at it again. Anyways, the system is full and bled out nice. Is there any standard method of elimination here to determine how the air is getting in? I'm pulling my hair out here

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Instead of pulling your hair out, you can rebuild the cylinder and caliper inexpensively. Do one, then the other. Take your pick as to which one to do first.

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Yeah I suppose thats what I'll try, I'll call the dealer tomorrow and see what the kits cost. I just hate using the iny, miney, miny, moe method of repair haha

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just to be absolutely positive (you keep referring to the 'oil' in your brake system) - you're using DOT4 brake fluid, and not something like mineral (or motor) oil, right?

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Did you check and confirm that the master cyclinder push rod has a little vertical play? You should be able to wiggle the clevis just slightly. If it is tight, you need to adjust it. Many people have adjusted the front brake lever or foot pedal screws to move the lever to a position they like and fail to ensure a little play exists. This can cause the piston in the master to cover the port and possibly contribute to your issue.

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Did you check and confirm that the master cyclinder push rod has a little vertical play? You should be able to wiggle the clevis just slightly. If it is tight, you need to adjust it. Many people have adjusted the front brake lever or foot pedal screws to move the lever to a position they like and fail to ensure a little play exists. This can cause the piston in the master to cover the port and possibly contribute to your issue.

Interesting theory. Well, I haven't checked that per say. Without looking right now I can say that I keep my rear brake lever adjusted to the looser side. I like a good bit of play in that lever, I cant stand touchy brakes. But I suppose this problem you mention would come from a tight lever setting though?

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While I haven't had it happen in awhile, I've had the rear brake go out as you described. Yamaha used to sell a replacement kit where you pulled out the the whole plunger from the master cylinder and installed the new one...easy to do. As for bleeding, I bought a cheapo vacumm bleeder, makes the job of bleeding brakes much easier...especially doing it solo.

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