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Discussion Starter #1
Hi,

So i need to press brake lever like 3-4 cm before it starts to brake, if i press brakes with full force the lever can almost touch the handle. At first i thought that the problem was hoses, so i changed them to HEL braided ones. But braking didn't change at all. Also the master cylinder seems ok, when i was bleeding the brakes after changing hoses i've noticed that it squirts brake fluid at around 2cm of lever travel. "Elastic band" method helps only for a while, i win ~1cm but after a while it goes back. Brake pads also seems good, i compared them with new ones and there is a lot of the pad left.

What could be the problem?
 

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I'd make sure your fluid is topped up in the reservoir first.
Next if you have factory levers try adjusting the dial settings. I can't recall if the it decreases pull required if you dial them up or down.
Next I'd check the pads for thickness (I know you said you did). If they are shot - replace them!
It could also be an air bubble somewhere which would require more bleeding.
Lastly if there's no leaking anywhere, and the hoses are not expanding (you said you replaced them), then my guess is the seals in the piston at the reservoir are leaking and may require a rebuild kit.

If in doubt have them checked by a professional. They are kind of important ;)
 

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Discussion Starter #3
Yeah, i think i will rebuild master cylinder. All the other parts seems ok. I've also noticed that if i leave brakes on for the night in the morning there is no grip even if the lever is fully pressed(when i depress the lever and press again the brakes are back). There are no leakes, so the only thing that could be "leaking" is master cylinder.
 

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Most Kaws seem to have a bleed nipple at the lever. I always manage to forget that one and scratch my head for a while.
 

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Discussion Starter #5
So, the update.

I've done a master cylinder rebuild. The rubbers on the old one seemed good and after the rebuild and bleeding nothing much changed :/ So me and my father investigated calipers. Apart from pins being a bit sticky, everything else seemed fine (we lubricated them).

But we have noticed that pistons travel a lot. Each piston moves ~0.5mm in and out when the lever is pressed and depressed. So the big part of lever travel is "used up" to move pistons without them creating any braking force. So that seemed to be the problem.

Is it normal? My father said that pistons normally don't travel that much, especially when lever is depressed.
 

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Discussion Starter #6
I think i've found a solution. Will try it in near future.

Quote from reddit:
The basic premise of the problem is that when you apply the brakes, the rubber gaskets in the caliper stretch to allow the pistons to move the pads agains the rotor and that because of this stretch, the piston is not able to slide out from the caliper. When you release the brake the rubber gaskets "grab" the pistons and pull them back into the caliper leaving a larger than normal gap between the pad and the rotor. Every time you want to apply the brakes, the pads have to close that excess gap before meeting the surface of the rotor, hence the increased lever travel before the brakes would actually engage.

It's actually really simple to solve this problem. The "Vincenzo method" mentions removing each caliper finding something slightly thinner than your rotor, inserting it between the pads, and compressing the pads onto the slightly thinner material so that the pistons can reseat in the gaskets.

I didn't bother with that, but what I did do was remove each caliper one by one, press the brake lever once with the caliper removed, then do a test fit of the caliper back on to the rotor. After doing this a few times with each caliper, the caliper would not slide on and off the rotor easily meaning that the pistons had reseated properly in the gaskets, and the gap between rotor and pad was very small as it should be. Now when I apply the brakes, the brakes engage within the first 25% of the lever travel and I can't squeeze the lever all the way to the grip even if I tried.

I hope this helps someone else.
Original post.

Also linko to Vincenzo method.
 

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Sounds like you may have found your issue, but I'll throw in my 2 pennies anyway. I had a track bike that was doing this exact thing, and it turned out that the brake pads had been contaminated with fork oil. An easy test is to remove one or both of the pads and hold a torch to the braking surface for a few seconds. The flame will pull any fluids to the surface if they're contaminated. If they're not, no harm no foul, it's just a little fire.
 

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Might be a good idea to service your calipers. Corrosion can set in behind the seals and this is what makes the seals grip the pistons harder..they then get pushed in to the disc but don't release fully. This wears the disc down and you have nice wavy patterns on your discs.

Seals are cheaper than discs.

Anilv
 

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Did the cousin Vinny method solve the problem? If not, I recommend tear down, clean, and new seals also. If you rebuild it, they will work.
 

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Discussion Starter #10
Did the cousin Vinny method solve the problem? If not, I recommend tear down, clean, and new seals also. If you rebuild it, they will work.
No, it didn't. I ordered repair kit for one of the calipers(the one that has more pistol back and forth travel) and see if it improves the braking. If it does i will replace another one.

The only good thing from this is that it forced me into the habit of using front andrear brake religiously, not because the front brake is not strong enough but because i can't trust it 100%, lol.
 

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No, it didn't. I ordered repair kit for one of the calipers(the one that has more pistol back and forth travel) and see if it improves the braking. If it does i will replace another one.

The only good thing from this is that it forced me into the habit of using front andrear brake religiously, not because the front brake is not strong enough but because i can't trust it 100%, lol.
While the majority of the braking power is in the front, it's always a good idea to use both - for a couple of reasons. 1. there are cases where the front wheel only can lock up which is not good. 2. front wheel braking in a turn is not good (well on our old bikes anyway...) 3. using both front and rear will provide more braking ability and get you stopped faster and in less distance than the front alone. There are other reasons as well, which is why most newer bikes have ABS and linked braking.
 

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Discussion Starter #12
So i disassembled left caliper(that was more problematic), it was really hard to take out the pistols (had to put the braking line back and push them out that way). The pistol and the cylinder were coated with some black and white stuff. Rubber seemed fine. I've cleaned everything and put everything back. And it helped! So i've decided to clean right caliper as well, it also was messy. And it helped even more! There is still quite a lot of "free travel" but now it's way better than it was before. It's impossible to push the lever all the way to the handle. I probably need to bleed the brakes more, there is probably some air left but i'm way more confident using my brakes now.
 

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Did you clean the groove the seal sits in? Corrosion builds up there and that pushes the seal harder onto the piston.

Great that you're progressing!

Anilv
 

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Discussion Starter #14
Yes, i did clean the groove, there was some corrosion. After all the cleaning I was able to move pistol with my hands, so the seal wasn't pushing too hard.
 
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