Posts
14
Joined
3/2/2022
Location
AU
Edited Date/Time
4/16/2022 8:30am
I have a problem with my front brake (yz250f 2017). When i brake into big braking bumps or brake right after landing from a big jump, front brake loses pressure and i need to pull it more. To be more precise, the enganging point of the brake change, the lever gets closer to the bars in order to stop than when i brake into smooth ground. I changed fluids,pads and installed a master cylinder rebuild kit, the brake became stronger overall , but the problem remains. Disk doesnt looked warped. I think it may be a line thing.
I also have a little brake fluid seep from the master cylinder cover after a day of riding. I ordered a SS front brake line and a new diaphragm for the master cylinder. I’m positive I’m not over filling the master cylinder.
I am hoping my problem is caused by a worn front brake line.
YouTube has tutorials if you don’t know the correct way to do it..
The lever is fine until 2 laps into my moto, the lever looses a little free play. The free play stays the same until around 15 minutes. I'll come into a turn and the lever gains a lot of play and goes soft.
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I have bleed the brakes many times, changed fluids, zip-tie overnight.
One time i zip tied the lever fully pulled in, i noticed that it leaked fluid from the master cylinders banjo bolt.
Motul 600 is a high quality fluid that works very well. I was 45 years old before i learned that not all brake fluids are equal.
As far as bleeding there are all kinds of tricks out there that you can look up. i would start with making sure there are no high points in your brake line that allows tiny air pockets to hide. you may have to remove the rotor and let it hang over night. i do that occasionally and tap on the line with a paint stick trying to free any possible air bubbles...
You mentioned you saw that the line leaked from the banjo bolt when you held the lever in overnight. That should not happen.
Replace the line banjo bolts and washers. Should be good to go after that.
Also when i put new fluids, didnt notice any difference. I dont think it is caused by boiling fluids because it can happen in the first seconds of the moto.
Front brake compressor
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Pit Row
But just to let you know, bleeding by pumping it up and cracking the bleeder will not always work on a front brake. Especially if you have emptied or cracked open the line. You have to use a bleeder pump you can get at the autoparts store and push the fluid from the bottom up and out of the master cylinder with the cap off. One that sucks from the new fluid bottle and pushes the fluid out. It is messy but that is what it takes.
If you don't have a suction bleeder tool you can do this but it takes a while, maybe a few hours. Take off you wheel or take your caliper off the rotor and let it hang straight down. Get the kink out of the line. With the pads in the caliper, hit your front brake and squeeze the pads together. Take off the your master cylinder cap and make sure it is full of fluid. Then simply crack your bleeder at the caliper and let it drip. It will drip rather slow if at all but just leave it like that, it will eventually do something. You will end up leaving it like that for a long time, maybe hours but check on it about every 10 minutes and make sure the master cylinder doesn't run out of fluid and suck air down the line. It will eventually push the air bubble out at the top at the master cylinder.
The reason you squeezed the pads together is in case you missed the master cylinder running dry. If it does, you can fill the master cylinder half way and push the pads out with a screwdriver (with the bleeder closed) and it will push the fluid up and out of the master cylinder and the air along with it.
It sounds like a lot but it really isn't that bad. I had to change a line once and like it sounds with you, I had hours of messing with it before I finally done this. Last resort and last great frustrated effort but it worked for me.
For the brake bleeding discussion here- my company did a study of brake bleeding for a motorcycle manufacturer and had clear acrylic master cylinders made to see the air bubbles in the master cylinder bore during the bleed process. We noticed that a large air bubble can often get stuck (due to surface tension of the bubble) inside the piston return spring, and because the bubble was larger than the coil spacing it was nearly impossible to remove it by normal bleeding. We found that 'flicking' the lever would agitate the bubble momentarily into many small bubbles that could gradually one-by-one escape the space between the spring coils. Very interesting problem that you would never realize.
Sounds like there is some play
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