Wanted: Disc Brake Guru

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#1
I have a '91 KDX 250. I have very mooshy brakes. So much to the point that my lever meets the grip before really starting to work. I ran a thread a couple months ago and got some advice, however this problem is not going away.

I have Replaced the caliper with a brand new one from Kawasaki, replaced the brake line with a steel braided one, put new piston and seals in the master cylinder, and bled the sucker for many hours using the following methods:
1. The old fashion way (depress the lever - open the bleed screw)
2. Reverse bleeding (compressing the piston and pushing the air out the resivor)
3. Using a mighty vac hand pump
4. Raising the caliper above the master cylinder then #1

I recently became convinced that it's not air in the system because when I tried to reverse bleed I had to use C-clamps to press the piston in. After doing so (with the clamps still in place) I gave the lever a pump and it was rock solid. I cheered ...reasembled it...pumped it back up and it was mooshy.

I took it to a motorcycle repair shop. The mechanic noticed that the side of the caliper closest to the wheel has to travel quite a bit before fully engaging and by that time the brake lever hits the handle bars.

My rotor is 3.2 mm thick I don't know the wear limit, my brake pads are brand new. Why isn't the caliper holding the pads snug against the rotor even when not being activated by pulling the brake handle. Isn't this how they usually work. Initially I thought it may be a defective caliper but nothing has changed between the old and the new making me think it's not the problem.

The last thing I tried was leaving my bike out in front of my house but no one would even steal it. Is there someone out there who has conquered this beast?

Severely frustrated, 300 dollars poorer, and 40 hours lost.:(
 

DougRoost

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#2
My riding buddy had this happen to his '99 XR250. His bike actually sprayed him with brake fluid on a check ride around the neighborhood (from up by the master cylinder). I replaced the washers on the banjo fitting and bled it but it was still dead. I inspected the master cylinder as well as the hose and all seemed fine. I removed the wheel, double checked that the pads were properly seated in the cailiper (because nothing else made sense). Finding nothing out of the ordinary I reassembled it.

Then I just bled the **** out of it. I mean we just did it the old fashioned way, him squeezing a couple times, holding, then me opening the screw to bleed. We repeated this until I put in an entire bottle (small) of Castrol LMA fluid. In the end it seemed the same, but when we closed it up and dropped it back off the stand the brakes were perfect and have been ever since. Go figure. May yours be so simple.
 
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#3
I stopped buying those little containers about 50 bleeds ago...I'm working on the 1 liter container now and have a pretty good dent in it! I wish it were that simple. Since the brake was mooshy when I bought the bike I don't know what led up to its demise. I wish I did.
 

Sage

dirtbike riding roadracer
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#4
if you bleed it and don't roll the wheel does it stay hard? then when you roll the bike the brake goes mushy? if so it sounds like you have a bent rotor.
 

G. Gearloose

Pigment of ur imagination
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#5
Perhaps check that the caliper is floating nicely on its pins, the retaining clip is not causing any compression on the caliper piston, and check the pads for flatness (uniform thickness); and the rotor as Sage suggested. Maybe a front wheel bearing or spacer is 'non-standard'. Do you top off the master resevior until it spills a little when you put the cover on? Is the cover's expanding boot torn?
 
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#6
Sage - I've never been able to achieve a hard feel with the caliper on the rotor. The only time I did was when the caliper was off the bike with the c-clamps in place. This is when I figured out the system was holding pressure. If there was air in the line I think it would still feel mooshy. It seems to have to do with the lateral alignment of the rotor in the caliper.

G - I was thinking maybe the wheel was shifted from right to left (sitting on bike) which would cause the rotor to come away from the wheel side of the caliper. Maybe a funky wheel bearing or spacer could do this. Is the rotor symetrical? Could it be flipped changing the alignment? If I can find a way to shift the rotor to the right (towards the wheel) then the play would be taken up on the wheel side of the caliper. This would then shift the play to the piston side of the caliper which would be taken up by pumping up the brake. And since that side is fixed it won't back away.

Everything else checks out G regarding your other questions. Thank you guys for your replys. As you can probably "hear" in my writing, I'm quite frustrated!

If you guys have any other ideas why this alignment could get off please let me know. I'll try turning that rotor around if it looks like it will help. I'll keep you posted. Thanks again

Renewed hope!
 

Sage

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#7
is the rotor centered in the caliper?
 
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#8
Sage - just came in from the garage and saw your post. I'm not sure about the rotor in the center...to the eye it apears so, however I just found something new. The casting of the caliper stinks.

The area which contacts the back of the wheel side brake pad is shaped like three finger-like projections - all evenly spaced. The articulating surfaces of all three fingers are not in line. I took the caliper off and held it up so that the pad was resting on the middle finger and I could "teeter totter" the pad up and down so that it would contact one of the side fingers and then the other.

It is physically impossible to make all three touch at the same time without significantly flexing the casting or the brake pad. When it is touching two of the fingers I can slip a .022 feeler gage between the pad and the third finger. Thats about a half a mm. Do you think that's enough to cause a severe mooshy problem? My old caliper didn't do that. I replaced it because I went to rebuild the old one and the cylinders were scorn.

Well the saga goes on...guess I'll probably have to send this one back and wait another three weeks to get a new one. Thanks again for your continuing interest.

I just want to ride! :scream:
 

gnarlykaw

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#9
how true?

well, I dont want to come off like a professor of hydraulics, but, all the parts must be perfectly squared to the rotor. the rotor must spin perfectly true, with no more that .003 runout. the caliper must pull into the rotor SQUARE.
the frt. whell must not have any bearing play, side to side or lateraly. if you can get a good holding pressure with the c clamp installed, its not the hydraulics that is at fualt, but the mounting, tapered friction material, or a rotor run out of some nature. class dismissed.;)
Rick
 
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#10
Well it sounds as if the mounting arrangement being off-kilter is your problem. Have you determined what part is at fault? I guess you are saying the calliper, as long as you are sure.

Just a point when disc rotors warps the metal gets too hot & expands, either they get a buckle in them or the outside gets bigger than the inside they can ‘cone’ - the outside moves to the left or right. This will flex somewhat. Check the rotor is flat with a straight edge to be sure.
 
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#11
I ride a KDX 250, and the brakes have always had a slightly mushy feel to them, especially compared to a Honda. I can get a reasonably solid lever with very careful and diligent bleeding, and the brakes are strong enough to do stoppies, but the lever has never been, nor will ever be, completely solid. It is just the nature of the design. I believe the master cylinder piston is too small in relation to the caliper piston, resulting in a mushy lever. The master cylinder mount and the caliper also have a small amount of flex in them, adding to the mushiness. Kawasaki is not known for having the best brakes in the industry.
 
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#12
I do appreciate all responses, however gnarlykaw man, you've got to read all the material before you present it to the class. We've already stated that the hydrolic seems to check out because of the c-clamp theory. And as far as the rotor goes - I do believe that my old frisbee can't be helping the situation but did you read about the flex in the caliper due to an unevenly machined surface with a variation of .022 inches on the clamping surface. Do you have any experience with this?

A very small variation in rotor runout can be accomadated by the floating side of the caliper, however there are no checks and balances for flex in the casting due to the uneven surface. My bet is this is the more contributing factor.

I really do appreciate your response! Anything that gets me riding again.

I'm sending the caliper back. After I get another one and rule out at least the flex problem then I'll let you all know how it's going. Thanks again for your input. And class isn't dismissed until I'm riding again!

Spanky - Do you have to abandon all four fingers on the handle bars in order to come to a stop? I do, If not the brake lever will bottom out on my nuckles and I may eventually come to a stop with the help of a tree. Where I ride is mostly hilly technical single track and it's pretty difficult coming down those hills like that. That piston in the master cylinder is about half the size of my pinky and it's pushing two "hockey pucks"...I'm no engineer but...In any case, next time you reverse bleed your brakes, just for kicks throw a c-clamp on there and dream about how it could feel with out all the flex. It's always good to hear from another owner of Kawi's stepchild, the KDX 250.

Anxiously waiting...:D
 
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#13
I'm not a guru of any sort, but I gotta give my $.02.
Fisrt I think there two distinct issues -Is it 1)a mushy soft feeling or 2) too much lever travel? I think they are two different problems. Regarding the mushy feeling - if there are no leaks, and the feeling stays the same for a while if you hold the lever then I'd say it's air in the system or stretchy brake lines -the stainless steel lines should have fixed that. If the lever slowly compresses as you hold the squeeze, I'd say the master cylinder piston is leaking by.
If it's just lever overtravel - here's my guess. Not sure about KD's and such but other brakes I've had use a "twisted O-ring" to return the brake piston when the pressure is released. As the brakes wear the O-ring slides a little in the cylinder, adjusting the brakes. I'd guess that for some reason your twisted O-ring isn't slipping(There's a song title for someone), and your brakes aren't adjusting as they should be. Good luck! Keep us posted - my new to me 97KDX2220 has a soft feel that I think I'll have to get used to I guess. Maybe when my manual shows up it will have some suggestions.
 
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#14
Finally

My brake's finally working. I returned the caliper with the poorly machined surface and got a new one. My new one bore the same part number but looked as if it was re-engineered. The casting was beefed up to prevent some of the flex in the caliper. Not only that but the overall finish was less porus and looked better. Anyhow the new one worked better.

The new system still allowed the brake lever to travel too close to my fingers remaining on the grip so I replaced the lever with a shorty with a more outward bend. Now it's as close to perfect as you'll get out of an old KDX from what I understand.

Just wanted to let all of you who responded to my cries for help that I've already put in about 15 hours since I got it together a couple weeks ago and it's working great. Thanks for your input!

T.J.:)