Front brakes only work When i'm not moving?

[rpruett]

So I was out for a ride today and long story short, took a turn too fast and laid bike down on the left side. There was no substantial damage, only a bent bar-end on the left side and some scratches. I killed the engine as soon as I could and then stood the bike back up and put down the kick stand. I dusted myself off, gave the bike a good look over and everything seemed fine, phew. I hopped on it, started down the road going slow to make sure it wasn't going to fall apart and the first turn I got to, I went to hit the brakes and there was no pressure in the front lever at all....so I pulled over, and stopped and magically the pressure came back... I lifted the front wheel off the ground and spun it by hand and tried to squeeze the lever and it was limp, let the wheel stop and the lever is firm again instantly.

So it looks like the front brake only works while the front wheel is stopped. Thoughts?

 

[Umyaya]

Sorry to hear about that. glad you are okay. What happened? Target fixation? Locked front brake? Gravel?

 

[Blackfin]

Check your brake lines and front rotors (esp the one of the side the bike went down on.) A slightly bent rotor could push the pistons back into the calipers leading to the feeling you're seeing in the lever. If a line snagged or was pinched during the off it could have a pinhole leak as well.

Don't ride the bike until you figure this out.

 

[rpruett]

We came up over a hill and the road came to a T so it was just an all out, slam on the brakes with the rear tire sliding. My roommate in front of me barely got stopped but his bike is lighter. There was a guardrail at the T so when I slowed down enough, I went to lean and try to turn and dodge the guardrail and that's when the rear tire slid out from under me.

 

[rpruett]

The rotors on first glance looked fine, you'd think a bent rotor would still work when rolling but just be jerky or vibrate a lot. A friend of mine suggested a wheel speed sensor sending the wrong info to ABS but to my understanding, the 2013 FZ8 don't have ABS in north America.

 

[Satan666]

...... bike down on the left side... no pressure in the front lever at all....stopped and magically the pressure came back... I lifted the front wheel off the ground and spun it by hand and tried to squeeze the lever and it was limp, let the wheel stop and the lever is firm again instantly.

So it looks like the front brake only works while the front wheel is stopped. Thoughts?

Most possible that when bike was down on left side, handlebars were full lock right and front brakes higher than handlebar brake reservoir. During the minutes the bike was down, air migrated into the lines and possibly caliper cylinders. When you pumped the lever while upright, most of the air was purged giving you some resistance at the master cylinder.

All brake rotors exhibit some runout whether it is side-to-side from mounting, bent/warped disc or disc surface differences from machining or heat warping. When the disc rotates, it pushes the brake pucks into the caliper cylinder, away from the disc surface and reduces rotational friction, wear and heat. This is good, as long as the displacement is minor. It is part of the amount the brake lever moves before it gets solid and the braking takes place..

If you have air bubbles in the caliper cylinder cavity which are trapped and do not flow upwards back to the hose, it will require additional travel of the brake lever to make up the distance and pressure of the pad against the disk. When the disc is not rotating only the air bubbles need to be compressed, giving you some feel at the brake lever.

Check for leaks and do a competent brake bleed (with the proper fluid) and see if the condition persists.

 

[rpruett]

When the disc rotates, it pushes the brake pucks into the caliper cylinder, away from the disc surface and reduces rotational friction, wear and heat. This is good, as long as the displacement is minor. It is part of the amount the brake lever moves before it gets solid and the braking takes place..

If you have air bubbles in the caliper cylinder cavity which are trapped and do not flow upwards back to the hose, it will require additional travel of the brake lever to make up the distance and pressure of the pad against the disk. When the disc is not rotating only the air bubbles need to be compressed, giving you some feel at the brake lever.
.

This makes the most sense. I didn't realize the pucks get pushed backwards further when riding. I did quite a bit of bleeding last night with no luck so I'm betting I'll have to flush the system to get all the air out. I'll do that in the next couple of days and report back.

Thanks!

 

[rpruett]

I had some time to work on it today before the rain swooped in. Here's what I did:

Took the top off of the master cylinder
Opened both caliper bleeder valves
Unhooked the hoses that go to both calipers
Pumped the lever until no more fluid came out of either side
Took both calipers off and squeezed the calipers to get any flood inside of them out (also turned them upside-down and such).
Put everything back together and proceeded to bleed them again.
Bled both sides until no air was coming out.

And after all of that, same outcome
Still works when I'm just sitting on the bike and stops working as soon as I start moving. It just strikes me odd that the lever isn't spongy at all. its a full handle of brake when sitting still, then doesn't catch at all when the front wheel spins.

 

[Satan666]

If at first . . .

I am assuming that your final bleeding procedure did not confirm to your first explanation. Final bleeding should be done by:

1. Keep the M/C reservoir at or near a full level at all times.
2. Tighten both caliper bleed screws and attach clear tubing to the screw being bled.
3. Pump brake lever until compressed.
4. Crack the valve on the far caliper and allow fluid to bleed until lever fully depessed.
5. Tighten bleed valve then release and re-compress lever.
6. Repeat step 4 until released fluid clear.
7. Repeat steps 3-6 on near caliper.

The caliper screws should only be cracked open slightly while under pressure or by using a hose attached vacuum tool (which requires close monitoring of remaining fluid level in the reservoir).

First, confirm that the bike is not Eurospec and does not have ABS. The brake line from the M/C goes to a splitter, then directly to the calipers

Next step, raise the front wheel off the ground, rotate it and observe each caliper and determine that the rotors do not have excessive > .010" side-to-side run-out.

Their is a miniscule possibility that when you dropped, a casting maggot or piece of detritus from your master cylinder reservoir worked its way into the master cylinder piston and lodged between the seal and spring. When back pressure from the rotating caliper hits the master cylinder piston, it exposes and hangs on the relief port. However; that would not explain why, when the wheel stops the cylinder instantly regains compression. If such is the case, either a cylinder kit 2D1-25807-00-00 or replacement master cylinder would be in order.



My next step would be inspection of the M/C and piston. Hope you have a good snap-ring tool.

 

[Blackfin]

And after all of that, same outcome
Still works when I'm just sitting on the bike and stops working as soon as I start moving. It just strikes me odd that the lever isn't spongy at all. its a full handle of brake when sitting still, then doesn't catch at all when the front wheel spins.

So if you're sitting on the bike on a flat, level surface and squeeze the lever, it's fine. What happens if you scooch forward using your feet about 1/4 of a front wheel revolution? How does the lever feel? Scooch another quarter and repeat. Repeat this until you've gone at least a full rotation. Do you note anywhere where you sense losing the lever?

You can also try starting the engine and, while slipping the clutch and moving forward very slowly, hold the brake lever to put some drag on the front brake. As the bike rolls, do you get a sensation that at certain point(s) of rotation of the front wheel you get more braking effect than others?

 

[rpruett]

I've looked at the calipers and just eyeballed them and they seemed fine, but obviously looks can be deceiving. This is probably the next step since I don't have spare M/C assy laying around to test with, nor do I want to mess with pulling out Section 2 of your image (although I did take the handle off to make sure that the push-pin wasn't in wonky or anything). Does anyone have a SOLID way to check the rotors without having to take the wheel off? Maybe just holding something straight across the whole rotor would show a dip/bow? I don't have a stand to leave the bike up on (without asking around to borrow one) so the best I can do is prop one side of the forks up on a cinder block just long enough to spin the wheel and I'd rather not do that again unless I absolutely have to. .01" is a 1/4 of a millimeter so I doubt I'll be able to see that just by eyeballing anyway. If I find a stand, I'll take the wheel off and lay both rotors on a table (probably a few surfaces to be sure) to see if either one isn't completely parallel with the surface of the table. If one is bent, is there anything I can do to temporarily work until I can get a new one ordered? Is it common to be able to just bend it back? Rubber mallet gently? Or could I just remove it and plug the other line to only use one rotor and just take the hit on braking power?

I'm just spitting out ideas, most of which are probably not ideal and/or safe....

 

[rpruett]

Over the weekend I had some time to work on this some more and it turns out, only the right-side rotor is bent and is pushing the caliper inwards causing the handle to have to move further to make contact with the brake rotor. So I got out my handy pair of vice-grips and an adjustable wrench with a piece of PVC pipe as a breaker bar and put aaaallll my weight onto it to attempt to bend it back. I made a little progress but only bent it about halfway back, it's probably about half a millimeter out now. this at least made the bike ride-able for the time being. guy in our local riding club has a set of rotors with ~150 miles for ~$80 so I'm gonna snag those and he said he'd put them on for me. Things are looking up again!

Thanks for the help folks!

 

[triplez]

Dafuq did I just read