Posts
330
Joined
8/25/2008
Location
Denver, CO
US
Edited Date/Time
5/21/2017 12:33pm
I recently purchased a used 2012 KX450F after a long time on the sidelines, and I was super excited to take it out to the track and remind myself how bad I suck. Everything was going great until I got the nerves to start hitting some jumps again, and that's when I noticed it: a heavy drag on my front brakes causing a grinding noise. I pulled off and threw the bike on the stand and sure enough it was struggling to make a couple revolutions when I spun it.
Here's what I've checked/done:
Checked the disk: it doesn't appear to be bent/have any issues
Made sure the lever wasn't depressing the MC piston while at rest
Rebuilt both the caliper and the master cylinder and changed the fluid/reverse bled the system
The wheel spins longer now on the stand, but there's still a heavy drag that's audibly noticeable on the track. Obviously I want to try and get this resolved, I'm worried about lock-up if everything gets hot enough. Any ideas or suggestions?
Here's what I've checked/done:
Checked the disk: it doesn't appear to be bent/have any issues
Made sure the lever wasn't depressing the MC piston while at rest
Rebuilt both the caliper and the master cylinder and changed the fluid/reverse bled the system
The wheel spins longer now on the stand, but there's still a heavy drag that's audibly noticeable on the track. Obviously I want to try and get this resolved, I'm worried about lock-up if everything gets hot enough. Any ideas or suggestions?
Paw Paw
My fluid level looks correct, and the calipers were completely pulled apart/cleaned and lubed per the service manual.
The pads are worn, though...I guess I never even considered that. I even thought worn pads might help a tad with clearance. At the expense of sounding like an idiot (guilty), how would that negatively effect operation? More difficulty returning the more the caliper is pushed out?
Paw Paw
Possible air in the line.
Brake pins have notches, holding the brake pads up.
The Shop
Put a squirt of this grease in the sliding pin holes, its a silicone based grease from napa for brakes, anything else will swell and ruin the rubbers. Most one year old bikes are in need of this cleaning and greasing.
If that doesnt work, you have to take the caliper apart, remove the pistons, clean out the seals and grease the pistons with the same sil glyde grease. I have my so clean and properly greased you can put the pistons in and out with your fingers like nothing.
As you can guess im very picky about my front brake.
In regards to some of your thoughts:
The wheel bearings checked out, there doesn't seem to be an issue there.
The brake pad pin was replaced when I took apart, cleaned, resealed, and lubed the caliper, so I don't think that's the issue.
Checking the lever for free play was one of the first things I did, I was hoping I was going to get lucky.
Adam, I actually used Sil-Glyde when reassembling my caliper, everything seemed to be moving free and smooth so I thought I was in the clear.
I was pretty confident I got all the air out by reverse bleeding, I even ended up doing it again just to be sure. Feel is the same but function isn't, there's still some drag.
I just don't know, I'm stumped. I'm probably missing something, but I'm not quite sure what. Thanks again, guys.
1. The seals in the caliper will grow with time and keep the piston from releasing after compressed.
I noticed you said the caliper was cleaned, but you MUST remove the seals and put new ones because the mold grows under the seal where you can't see it, and it dries in there crusty. You scrape inside those groves when the seals are out and many times we get major crusties in the groves under the seals causing the piston to be held to tight. The way to check if this is the problem is to pump the brakes so the wheel moves slowly, then take a dead blow hammer and lightly tap the caliper on the outside, if your wheel then spins freely you know it's the piston/seals in the caliper. Don't slam the caliper and bend the disc, just a light tap will do it.
2. Tiny bleed hole in master cylinder.
I will pull the caliper back apart and inspect the guide pins again, but I polished those up and they appeared to be straight on initial tear down, and they seemed to be functioning properly/smoothly by hand. I didn't look at the actual fork lugs, though, so that will be something else to give a look.
Grabbing some new pads tomorrow, also.
Thanks again, guys.
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