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Air leaking out of front brake reservoir

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Hey guys, I had some front brake issues and went to bleed my brakes. After a pissed of 30 minutes of no luck I realized I had a tiny bit of air leaking out of my seal on the brake fluid reservoir.... I took the cap off and lightly sanded it down with 800 grit and made sure the seal is in there correctly and everything is clean, but I'm still getting air leaking out. Do I need a new rubber seal thing that goes in there?

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How can you tell if air leaks out, it leaves no visible traces as far as I know?

The reservoir cap is not supposed to be air tight, well it should seal around the edges, but there are 2 small grooves where it lets air in and out to let the diaphragm move.

I hope you did not remove the air channels.

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Most likely your issue is not with your reservoir cap and or seal. I believe the reservoir is designed to be sealed but will allow air in or out of the seal channel in certain circumstances. This does not indicate you are getting air into the closed chambers of the hydraulic system.

What are the symptoms that warranted the bleed procedure?

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When I bought my KX450, the front brake was quite squishy, and the lever could easily be pulled to the handlebar and produce very limited braking force.

I bled the brakes 10 times but it would only temp fix the issue. Ended up buying the master cyclinder rebuil kit for about $40 from the dealer. The seal was slightly leaking air every time i hit the lever.

Install was easy and braking was suprb after! :thumbsup:

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i didn't sand enough material off to cause it to be air tight.

i could see the air when there was a film of brake fluid in between the cap and reservoir.

i cant build up enough pressure after bleeding it to have a firm pull on the brake lever, it always bottoms out.

so u guys think i need to rebuild the master cylinder?

thanks for all the help!!!

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I have seen front brake systems that you can swear needs a M/C rebuild. Then you bleed it rapidly a couple more times and a big bubble comes out and you´re like, ***!?

They are notoriously hard to bleed. Back bleeding might help, but not always. When back bleeding you remove fluid from the reservoir and push it backwards through the bleed nipple with a big syringe or similar. That´s the way bubbles like to travel.

If bleeding the regular way, put a hose on the nipple (not yours :thumbsup:) and route it to a catch bottle. This will let you bleed it without worrying about catching the fluid. Bleed it rapidly, or the bubbles will start to rise in the brake line again.

If this will not help, rebuild it. It is possible to get a firm feel, even though everyone bags on Kawi brakes.

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When I bought my KX450, the front brake was quite squishy, and the lever could easily be pulled to the handlebar and produce very limited braking force.

I bled the brakes 10 times but it would only temp fix the issue. Ended up buying the master cyclinder rebuil kit for about $40 from the dealer. The seal was slightly leaking air every time i hit the lever.

Install was easy and braking was suprb after! :thumbsup:

The rebuild kit very well may have fixed your problem, but your understanding of the system... I believe is off.

http://www.google.com/imgres?q=brake+hydraulic+master+cylinder+motorcycle&um=1&hl=en&rlz=1G1GGLQ_ENUS261&tbm=isch&tbnid=dAtXzZChBuq0jM:&imgrefurl=http://www.ducati.ms/forums/80-hall-wisdom/29044-clutch-master-cylinder-adjustment.html&docid=qkX3LBwT49KjqM&imgurl=http://www.ducatisportingclub.com/images/dsc/shazaam/ReservoirPorts.jpg&w=445&h=330&ei=NqGdTqmKEOikiQLviMTsCQ&zoom=1&iact=rc&dur=461&sig=109759364702787105363&page=1&tbnh=117&tbnw=158&start=0&ndsp=15&ved=1t:429,r:1,s:0&tx=57&ty=78&biw=1024&bih=611

In order for air to leak from the system there needs to be air present. By design a hydraulic brake system doesn't work well with air in the system. Air will not enter the system if hydraulic fluid covers the supply port and return port. A malfunctioning piston seal in the master cylinder will not allow air to enter, only allow fluid to pass, which will certainly cause poor brake performance. (spongy lever)

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I have seen front brake systems that you can swear needs a M/C rebuild. Then you bleed it rapidly a couple more times and a big bubble comes out and you´re like, ***!?

They are notoriously hard to bleed. Back bleeding might help, but not always. When back bleeding you remove fluid from the reservoir and push it backwards through the bleed nipple with a big syringe or similar. That´s the way bubbles like to travel.

If bleeding the regular way, put a hose on the nipple (not yours :thumbsup:) and route it to a catch bottle. This will let you bleed it without worrying about catching the fluid. Bleed it rapidly, or the bubbles will start to rise in the brake line again.

If this will not help, rebuild it. It is possible to get a firm feel, even though everyone bags on Kawi brakes.

Right on! Along with all the knowledge mentioned here one more trick I also use, open the bleed bolt and allow fluid to run free for about three times the total volume of the system, maintaining a full reservoir while leaking out the bottom. This has the potential to remove all air quickly. I usually bounce back and forth between several methods just to make sure it is good.

If the master cylinder has issues after bleeding, usually you will have a firm feel. If you keep pressure on the lever you will notice the lever slowly moves towards the direction you are pulling. (toward the bars) This tells you the seal is leaking fluid back into the reservoir. Same on a car, if you step on the pedal and it is firm initially then slowly is depressed to the floor, this tells you the seal is leaking.

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Thanks for the tip. My lever would slowly depress to the bar when pulled in and held before.

Leaking air / leaking fluid --- it doesnt matter to me. Rebuilding the master only thing that fixed my issue for longer than a few days.

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Funny story, I bleed and bleed that brake with no luck. Then a couple weeks later tearin down the track I accidentally reach for my front brake and I have full compression and perfect brakes..... air bubbles musta worked their way out??....:busted::ride::busted::ride::bonk::ride:

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Funny story, I bleed and bleed that brake with no luck. Then a couple weeks later tearin down the track I accidentally reach for my front brake and I have full compression and perfect brakes..... air bubbles musta worked their way out??....:busted::ride::busted::ride::bonk::ride:

They are hard to bleed for sure. I had a feeling there was a bubble still in there...

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