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Discussion Starter #1 (Edited)
Front brake not working properly!

I have a '95 model zx6r and one day I applied the front brake but the bike just kept rolling past the traffic lights. I had to use the rear brake to stop.

It didn't happen again for about a week but it's been getting worse and more frequently. Today it happened about 10 times as I was approaching intersections.

When it happens, there is no feel in the brake lever compared to when it's working properly I can feel the pressure on the brakes getting tighter the more I squeeze the lever. It feels like there's no resistance and the lever just bottoms out to a point where it can't move any further. I already changed the brake fluid and bled the brakes a few days after it started happening. I didn't notice any air bubbles coming out when I was bleeding the brakes so not sure what the problem is...

Could it be the master cylinder? How would I diagnose/fix it?
 

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Sorry, I just read the rest of your post, sounds like air is getting in there some how. Look for any leaks perhaps zip tie the brake lever down and leave it over night then check. Banjo bolts near the crush washers can leak, any holes in the lines, there are seals in the calipers and master cylinder which can fail. It is a fairly simple system. Squshie lever = air somewhere. Also always use a fresh unopened bottle of brake fluid. That shit is hygroscopic and any water it absorbs can lower the boiling point.

If they are fine in the garage then after some riding go to shit:

Sounds like you are boiling something (either brake fluid or water) this can lead to catastrophic failure.

If a good flush does not solve it (I would honestly do this at the same time as I do the fluid flush) take a look at the calipers and make sure nothing is binding.

I would definantly not ride until fixed.

Brakes keep you alive.

For fluid I like this stuff http://www.amazon.com/Motul-RBF-Racing-Brake-Fluid/dp/B000AURZ08

To flush I use one of these Mityvac MV8000 Automotive Test and Bleeding Kit:Amazon:Automotive

Undo res cap, suck both sides dry using vacuum pressure, then I like to suck a res worth through each bleed valve, finally the pump pump squeeze with corridanated bleed valve opening and the job is done in 15-20 min. If I zip tie the lever to the throttle and leave it over night it is rock solid the next day.

If they suck in the garage:

You have air somewhere. Either a leak, or some air you haven't found yet. Keep bleeding.

Ey3
 
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I have a '95 model zx6r and one day I applied the front brake but the bike just kept rolling past the traffic lights. I had to use the rear brake to stop.

It didn't happen again for about a week but it's been getting worse and more frequently. Today it happened about 10 times as I was approaching intersections.

When it happens, there is no feel in the brake lever compared to when it's working properly I can feel the pressure on the brakes getting tighter the more I squeeze the lever. It feels like there's no resistance and the lever just bottoms out to a point where it can't move any further. I already changed the brake fluid and bled the brakes a few days after it started happening. I didn't notice any air bubbles coming out when I was bleeding the brakes so not sure what the problem is...

Could it be the master cylinder? How would I diagnose/fix it?
If you don't know the last time the brake fluid was changed on a 20 year old bike, odds are good things are not up to snuff.

Four parts of the brake system deteriorate with age -- whether the bike is ridden or not. The calipers, the master cylinder, the brake fluid, and the brake lines them selves.

Both the master cylinder and the brake calipers use O rings to seal the pistons to the cylinder walls, and both are prone to failure over time. A rebuild of both front calipers, and the master cylinder is a good option, just for the increased confidence that would allow.

As the brake fluid ages, it draws water out of the air. That water will collect in the brake fluid, and due to the specific weight of the H2O being greater than the silicon based oil used for brake fluid, it sinks to the lowest point in the brake system.... the lowest edge of the lowest caliper cylinder is the most probable point. This might cause pitting which will destroy the seal. If caught early enough, refreshing the polished surface of the caliper piston bore (along with new O rings) may be enough to repair the caliper. Calipers also trap a lot of road munge, which can trap moisture, and accelerate corrosion/degradation of the caliper on the inside surfaces where it is difficult to see.

Master cylinders have the same issue.... and air bubbles in the system will work their way against gravity to the highest point in the system. A rebuild on the MC is going to improve your confidence in it.

The rubber jacketed brake lines are not a lifetime component.... the rubber looses strength over time, and allows for more expansion than it did when new. This makes for mushy feeling brakes, and potentially could result in a burst brake line in an extreme case.

Your options at this point are:

1) stop riding the bike and repair or replace the brakes.
2) keep riding the bike and eventually crash.

Me personally, I'd go with option 1.

From there,

Either buy the rebuild kits and do all of the brakes -- both front calipers, the master cylinder, the rear caliper and the rear master cylinder, as well as replace the brake lines with new ones. The new lines might as well be re-enforced with steel, or carbon fiber for better performance.

You could also possibly source replacement calipers from a later model bike and upgrade -- I'd still do the lines, and this might be more work than you want to take on.
 

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Sorry, I just read the rest of your post, sounds like air is getting in there some how. Look for any leaks perhaps zip tie the brake lever down and leave it over night then check. Banjo bolts near the crush washers can leak, any holes in the lines, there are seals in the calipers and master cylinder which can fail. It is a fairly simple system. Squshie lever = air somewhere. Also always use a fresh unopened bottle of brake fluid. That shit is hygroscopic and any water it absorbs can lower the boiling point.

If they are fine in the garage then after some riding go to shit:

Sounds like you are boiling something (either brake fluid or water) this can lead to catastrophic failure.

If a good flush does not solve it (I would honestly do this at the same time as I do the fluid flush) take a look at the calipers and make sure nothing is binding.

I would definantly not ride until fixed.

Brakes keep you alive.

For fluid I like this stuff Amazon.com: Motul RBF 600 Racing Brake Fluid: Automotive

To flush I use one of these Mityvac MV8000 Automotive Test and Bleeding Kit:Amazon:Automotive

Undo res cap, suck both sides dry using vacuum pressure, then I like to suck a res worth through each bleed valve, finally the pump pump squeeze with corridanated bleed valve opening and the job is done in 15-20 min. If I zip tie the lever to the throttle and leave it over night it is rock solid the next day.

If they suck in the garage:

You have air somewhere. Either a leak, or some air you haven't found yet. Keep bleeding.

Ey3
Bold section is actualy an awesome idea to see if there is any leaks! Thanks for mentioning that haha...Why havent i thought of that :coocoo
+1 on dont ride until fixed!
+1 on motul brake fluid (I used an overkill rbf660 and love it)
+1 on the mityvac (purchased at a local autozone. Bleeding brakes were never so easy until this tool!!)
+1 on keep bleeding until all air is out (if using the mityvac, until there are no more bubles being pulled from the bleeder nipple while keeping a steady 15-20psi in the lines)

I want to say the lines have air in it.
 

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If you don't know the last time the brake fluid was changed on a 20 year old bike, odds are good things are not up to snuff.

Four parts of the brake system deteriorate with age -- whether the bike is ridden or not. The calipers, the master cylinder, the brake fluid, and the brake lines them selves.

Both the master cylinder and the brake calipers use O rings to seal the pistons to the cylinder walls, and both are prone to failure over time. A rebuild of both front calipers, and the master cylinder is a good option, just for the increased confidence that would allow.

As the brake fluid ages, it draws water out of the air. That water will collect in the brake fluid, and due to the specific weight of the H2O being greater than the silicon based oil used for brake fluid, it sinks to the lowest point in the brake system.... the lowest edge of the lowest caliper cylinder is the most probable point. This might cause pitting which will destroy the seal. If caught early enough, refreshing the polished surface of the caliper piston bore (along with new O rings) may be enough to repair the caliper. Calipers also trap a lot of road munge, which can trap moisture, and accelerate corrosion/degradation of the caliper on the inside surfaces where it is difficult to see.

Master cylinders have the same issue.... and air bubbles in the system will work their way against gravity to the highest point in the system. A rebuild on the MC is going to improve your confidence in it.

The rubber jacketed brake lines are not a lifetime component.... the rubber looses strength over time, and allows for more expansion than it did when new. This makes for mushy feeling brakes, and potentially could result in a burst brake line in an extreme case.

Your options at this point are:

1) stop riding the bike and repair or replace the brakes.
2) keep riding the bike and eventually crash.

Me personally, I'd go with option 1.

From there,

Either buy the rebuild kits and do all of the brakes -- both front calipers, the master cylinder, the rear caliper and the rear master cylinder, as well as replace the brake lines with new ones. The new lines might as well be re-enforced with steel, or carbon fiber for better performance.

You could also possibly source replacement calipers from a later model bike and upgrade -- I'd still do the lines, and this might be more work than you want to take on.
nice write up :bigthumb:
 

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First of all, stop riding the bike immediately until this is fixed as this is a very dangerous situation.

If you feel pressure in the system build as you keep squeezing the lever it leads me to believe you have a small leak in the system somewhere. Examine the entire length of brake hose as well as the calipers and MC and look for signs of a leak.

Also, when is the last time you did a brake fluid flush and bled in new fluid?
 

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Discussion Starter #8
I just bled the brakes again and master cylinder as well this time. Again, no air bubbles were evident. Didn't see any obvious leaks either.

Come to think of it, all the times the front brake had failed it had been raining. Could it be dodgey pads/disc?

I recently replaced my rear pads and disc rotor which might explain why the rear brakes held up in the rain when the front brakes failed. The fronts still have some meat left in the pad so I haven't changed them yet...
 

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I had brake issues after a minor collision with a 4x4, I too thought somehow some air had gotten into the brake system after the collision, the weird part was, initially I had no feel when grabbing the lever, after giving it a few pumps, the pressure was back, then I went on my merry way. Just to be sure I tested the brake again, and the pressure was all gone. Repeated the earlier steps, pressure came back, after just rolling off a bit, pressure lost.

Tried to bleed the brakes and made sure no air was in the hoses. Still the problem was there. Was suspecting the master cylinder, in the end, it was the disc. Apparently one of the disc had bent during the collision, but was totally not visible, I think because it hit the rubber/tire of the 4x4, so no visible scratches (that's why it took so long to figure out the issue).

If you haven't checked your disc(s), you should try that. Just put a pencil tip against the disc and braced on the fender for a fixed position and spin the wheel, if you hear scraping or see a gap between the tip and disc when you rotate it, you've got yourself a bent disc.

According to my mechanic friend, it doesn't take much to dent a disc, some douche giving your bike a kick to the disc while parked, or some other biker accidentally hitting your disc while trying to park can be enough to dent it and cause braking problem.

Hope this helps in your troubleshooting. Having dodgy brakes is so frustrating.
 

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I just bled the brakes again and master cylinder as well this time. Again, no air bubbles were evident. Didn't see any obvious leaks either.

Come to think of it, all the times the front brake had failed it had been raining. Could it be dodgey pads/disc?

I recently replaced my rear pads and disc rotor which might explain why the rear brakes held up in the rain when the front brakes failed. The fronts still have some meat left in the pad so I haven't changed them yet...
Could you please give us as much detail as possible about exactly what is happening?
For example:
Date of last fluid flush and was the fluid fresh, unopened, new bottle?
Bike coming out of garage or on stands, what happens when you squeeze the lever?
When do the brakes go to shit? Is it you never have front brakes or they are great then stop working? More definition on stop working? Does the lever go all the way to the bar? Is the lever nice and firm, but the brakes don't slow the bike?

It is a really simple system you have to have one of these problems :
Air in there
Something preventing a piston from doing its thing
Something wrong with pads and/or rotors
A leak somewhere
Water in there
Some kind of crap blocking one or more of the tiny holes
There may be a few more options, but those come off the top of my head.

Ey3
 

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Just because you are incapable of getting air to come out does not mean there is not air in the system!

no lever then pumping on it gets you lever--air in system that simple........of course you could have other issues beyond that, but fix the root cause first!
 

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Discussion Starter #12
SYMPTOMS:

The rain was just coincidental and the problem is now consistent.

Normally, when traveling at speeds above 20 kmph or > 15 mph and rider suddenly squeezes front brake hard the front wheel should lock up and cause bike to fall over. When this is done on my bike, I can feel a small initial bite that will slow the bike down but then the brake lever just bottoms out and I can't get the brakes to bite any harder so the bike just keeps rolling forward even after the initial deceleration. This happens 100% of the time. It's good that the front wheel isn't locking up but very bad that I only have about 1/10 of the braking power I should have.

CLUES:

The bike has been dropped a few times and I have had to replace a set of clip ons and levers. Could it have dropped on the brake lever so hard it damaged the master cylinder inside?

The brake lever bottoms out an inch or two before touching the handle bar. I can just fit my fingers in between the lever and handle bar with the brake squeezed all the way down. But it definitely feels like it bottoms out on something when squeezed all the way down, maybe the piston inside the master cylinder can't go any further?

The brake fluid wasn't brand new but I don't think that's the problem because I used the same stuff in the rear brakes and they lock up fine at speed. I know how to bleed brakes properly and there is definitely no air in the system.

To make things more confusing, when I bled the brakes one side sprayed out brake fluid in a stream like it should but the caliper on the other side was a less powerful drizzle suggesting a partial block somewhere.

I will try the pencil trick to see if the discs are warped. Thanks again.
 

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The brake lever bottoms out an inch or two before touching the handle bar. I can just fit my fingers in between the lever and handle bar with the brake squeezed all the way down. But it definitely feels like it bottoms out on something when squeezed all the way down, maybe the piston inside the master cylinder can't go any further?
It sounds like something may be wrong in the master cylinder. Usually the brake lever will go all the way to the grip if you lose brake pressure.

You might want to disassemble the master cylinder to check everything out and possibly rebuild it with a rebuild kit.
 

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Discussion Starter #15
Okay the reason it stops an inch before touching the handle bar grips is because I have adjustable levers. If I adjust the settings, I can keep squeezing until it touches the throttle grip.

And yes, I would have changed the front brakes before changing the rear brakes if they were worn.
 

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SYMPTOMS:


To make things more confusing, when I bled the brakes one side sprayed out brake fluid in a stream like it should but the caliper on the other side was a less powerful drizzle suggesting a partial block somewhere.

I will try the pencil trick to see if the discs are warped. Thanks again.
Did you dissasemble and clean out the entire caliper? The dribbling fluid could definitely be a block somewhere. How much less of a stream is it than the other caliper?? Try removing and clearing the brake line, or try swapping it with the other to see if the dribble moves to the other side.
 

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Discussion Starter #18
Nah, not yet anyway. I only have so much time and money.

I ordered some new discs and pads. If that doesn't solve it I'll look at replacing brake lines with braided ones and then rebuild the calipers. Surely I will have proper functioning brakes by then, I would have spent the same on parts as what the bike cost me!
 
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