[SOLVED] Classic front brake issue?

[Charles]

Hello,

I started rebuilding my 96' 200 KDX. Since I had it four years ago it had some issues: Weird handling, forks and shock too hard, and spongy front brake with lack of stopping power.

The bike now has new tires, wheel bearings, got the front rim dents flattened, fork and shock serviced. I also cleaned and greased the swingarm and links bearings, and replaced one that was seized. Also replaced spark plug and gearbox oil, so it's ready to go exept... the front brake.

I rebuilt the master cylinder (whole kit, piston with gaskets and springs). Pushed the pistons of the caliper almost all the way out, cleaned everything, no leaks, no seizing. Bought some good Dot 4 brake fluid, topped it off, got the lever to firm up just a bit... and now I'm stuck.

Same as before, weak braking power, and with one hand I can quite easily make the lever touch the handlebar. Lever adjusting screw tightened all the way in.

What should I do now? I tried bleeding "conventional method" but I can't get a more firm lever. I moved the line so there is no loop above the master cylinder, tried zipping the lever overnight.. But no results.

I'd love to have a good braking bike. Also, I discovered there's quite a bit of play in the front disc, on the rivets between the inner and outer race. To the point where when applying brake and rocking the bike back and forth you can feel and hear it.

[Slick_Nick]

Why didn't you remove the pistons completely? Maybe you have a seal that's gone bad, allowing air in.

[ericr]

You probably have air in the caliper or line. Using the conventional pump, hold, crack and close method, some have had the best luck removing the caliper and having it higher than the master cylinder. I've had good luck using vacuum to draw out fluid and bubbles out at the caliper. You can use a vacuum bleeder for cars or I just used a large syringe drawing suction and then cracking the bleeder for an instant.

[KDXGarage]

25 year old dust seal and fluid seal may need to be replaced

disc is a floating desgn. it is made to float like that

[pumpguy]

Another issue to consider is the brake line itself which may be too flexible. If you have renewed all the seals and are sure all the air has been bled out, you might consider a braided stainless steel brake line.

[Charles]

Thank you for your answers.

I didn't buy the caliper repair kit or removed the pistons completely because I wanted to save some money. Maybe it wasn't the best idea as this is starting to be costly in terms of brake fluid lol.
Upon inspection the pistons were in perfect state and I guessed that if the seals were shot there would be brake fluid leaking, but I can be wrong on that, and they're old anyway.

Before handing out more money, tomorrow I'll get a syringe and I'll try vacuum bleeding with it, and also moving the caliper up. But I guess some day a caliper seal kit would be nice. I'll also consider the brake line to make it a complete rebuild.

I just bled the rear brake and it was so easy! Took me less than ten minutes to get the fluid crystal clear.

About the floating disc, is there a tolerance for the play? Or you just ride it until you feel something's wrong? I remember having a loose rivet on my GSXR1000 rotor, it made an horrible noise when braking. Ended up replacing the pair, I didn't dare to tighten it in a press.

[kdx633]

Air trapped in the upper banjo bolt?
1.remove caliper,pump until piston are well extended.
2.tie lever to handle bar.
3.loosen banjo bolt and fully compress the caliper piston with a clamp.this will expel fluid at the banjo fitting so be prepared.
4.tighten banjo and refill m/c.
5.you may have to crack the banjo one more time to bleed the m/c typically that just a little spurt.

[Charles]

Ok, so I got a syringe and some clear tube.

Tried bleeding at the banjo, no air there. Tried applying succion with the syringe at the bleed fitting, definitely got bubbles out. I also removed the caliper bolts and tried different positions with not much results.
I'm still bleeding and bleeding and still the lever is not hard like I think it should be.

This is getting on my nerves so that's enough for this evening. I mean, how hard should the lever be? I'll take it for a test ride to see I I have some braking power.
How are you supposed to brake on a KDX? Like, blocking the wheel with one finger?

[Slick_Nick]

Reverse bleed it. Pump clean fluid IN from the bottom bleeder, it will push air to the top. Don't overfill the reservoir when doing this.

[John_S]

I've had good luck with reverse bleeding but I don't think you'll get what you're looking for with a 25 year old brake line. That line is likely expanding, making it feel spongy.

[Charles]

Ok so I took the KDX for a quick ride this evening. First, the good news: This is not the same bike anymore. Rebuiding the shock and fork changed the handling dramatically, along with the new tires there is so much more grip than before. New spark plug got me a few more RPMs and new gearbox oil always feels nice.

Now the expected bad news: Front brake is just so weak! It's a little less worse than before but definitely not up to spec. Lever is spongy, with way too much travel, and very limited stopping power.

So I guess now I have to wait for the paycheck, and order some stuff! Let's say, a steel braided line, a bleeding tool of some sort and a caliper repair kit. Fun fact, I'm rebuilding the wife's Husqvarna and that bike has a front brake issue too! But the opposite. Lever is rock hard and even less power

By the way I tried reverse bleeding it but with not much success.. Maybe I'll try one last time before placing the order.

When ordering the braided lines, do you guys keep the stock lenght? I figured it is about 138cm. Or do you get rid of the loop above the master cylinder?

[Goat]

I used the sucking method with good success when I swapped to the KX125 setup extra long hose setup. It was bone dry from the salvage yard and needed some priming.
Theoretically I kept a constant vacuum on the caliber while filling the reservoir and exercising the handle.
It’s a little messier and less tasty than that sounds though.

[Goofaroo]

Ok so I took the KDX for a quick ride this evening. First, the good news: This is not the same bike anymore. Rebuiding the shock and fork changed the handling dramatically, along with the new tires there is so much more grip than before. New spark plug got me a few more RPMs and new gearbox oil always feels nice.

Now the expected bad news: Front brake is just so weak! It's a little less worse than before but definitely not up to spec. Lever is spongy, with way too much travel, and very limited stopping power.

So I guess now I have to wait for the paycheck, and order some stuff! Let's say, a steel braided line, a bleeding tool of some sort and a caliper repair kit. Fun fact, I'm rebuilding the wife's Husqvarna and that bike has a front brake issue too! But the opposite. Lever is rock hard and even less power

By the way I tried reverse bleeding it but with not much success.. Maybe I'll try one last time before placing the order.

When ordering the braided lines, do you guys keep the stock lenght? I figured it is about 138cm. Or do you get rid of the loop above the master cylinder?

The brake line is not your problem. If you haven’t done it yet you need to completely disassemble and rebuild the caliper. My guess is that you either don’t have the system bled properly or the caliper is not floating smoothly on it’s pins. Is the rotor glazed? Polluted with oil? Are you using ceramic or metallic pads? If so clean up the rotor and order some organic pads.

If you’re brake is as ineffective as you say the first thing I would suspect is that your caliper is not floating properly. Have you greased the pins and verified that the caliper is sliding freely?

There’s also a very small hole in the master cylinder reservoir. It is the return for the fluid. Poke a wire through it and make sure it is clear. Does your system ever seem to drag? Does the front wheel spin freely?

[Charles]

I think this is a bleeding problem.

I removed the caliper from the bike. The pins are clean, I greased them and they let the caliper slide smoothly. I pushed the pistons out, cleaned them and pushed them back. No leaks, when actuating the master they slide out easily.
Brake rotor is clean but has some play in the rivets. Brake pads are SBS SI pads, which are "sinter/carbon" and made for offroad.

No dragging from the front brake, wheels spins freely yes so the caliper works fine I believe.

I took the bike for a ride again tonight. Basically the lever is too soft, I have to pull it near the handlebar grip to feel some braking power. With my fingers out of the way.

Before buying stuff I'll bleed it again thoroughly with some help, so we'll put that caliper above the master and some PTFE on the bleeding fitting.

[ericr]

If the rotor is loose enough on the rivets, is it moving laterally like a warped rotor to compress the piston? If so, it would take a bit of lever movement before the pad bites.

[Charles]

I took some delay on that KDX, we got the bikes out and the wife's Husky broke! Gearbox out. Dropped the engine, turns out it's the gear selector spring. While waiting for the parts, I'll be back working on the KDX.

[Charles]

Well,
I put some PTFE tape around the bleeder fitting.
Removed the caliper and hung it upside down, tried to bleed it like that. Didn't work.

I'm running out of ideas.

[Molly's 70]

This is how I did it.

[Charles]

Ok, so I followed the procedure in the video above exactly. It did'nt get anby better. I can't find any bubbles in the circuit though. So maybe this is not a bleeding issue.

I'll try swapping the master cylinder, I have a 14mm with reservoir from a sportsbike laying around.

[Goofaroo]

Have you disassembled your existing master cylinder? After everything you’ve tried it seems like you’re getting the same result. Pull the m/c apart and see what you have.