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Discussion Starter · #1 ·
Hello

So I failed my MOT as there was excessive fluctuation in the front brake.. Pulled the calipers off to find both calipers had at least 1 stuck piston.,, So I was running on 6 pistons not 8. So I then bought some used calipers and picked them up, I have put them on the bike with near enough new sintec pads and bled the brakes from the MC and the left caliper then right caliper... I have tried bleeding over 10 times now but for some reason my brake pedal is gone very soft compared to how it was before.

When the lever was fine I had about 15-20mm of lever travel before brakes started grabbing whereas now I have about 50mm of travel before the brakes grab and if I try pulling the lever to the throttle grip, im only about 10mm away from the grip!!!!

Again, I have bled multiple times and it hasnt changed the lever travel... the lever adjustment is on 1 (hardest) whereas 6 has more travel than 1. What else could be wrong? I have new washers, new brake fluid and new pads..

heres a video
 

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Had this problem recently, tried everything. Ended up pushing pistons one by one out a fair way then pushing them fully back in. To get the air out of the system, then they were good again.
K4 levers same as you say.
 

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Discussion Starter · #4 ·
Had this problem recently, tried everything. Ended up pushing pistons one by one out a fair way then pushing them fully back in. To get the air out of the system, then they were good again.
K4 levers same as you say.
When you pushed them out, did you release the bleed valve or open the reservoir cap or just push them out and push back in that's all?
 

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Open reservoir cap ( make sure theres not too much fluid\spillage ), valves all shut. Push the pistons out ( not all the way ) then back in. I saw the air bubbles coming into the reservoir, could not get the air out any other way,bled & bled, cable tied the lever back overnight nothing.
 

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Discussion Starter · #6 ·
Open reservoir cap ( make sure theres not too much fluid\spillage ), valves all shut. Push the pistons out ( not all the way ) then back in. I saw the air bubbles coming into the reservoir, could not get the air out any other way,bled & bled, cable tied the lever back overnight nothing.
Okay sound i am going to try this first as I have tried everything else and cant get a stiff lever!

If that doesnt work i will just rebuild my own calipers and put them in as I dont know what else it could be... The rear brake works fine.. the front brake grabs fine but just the lever has too much play in it.

Thanks.
 

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Okay sound i am going to try this first as I have tried everything else and cant get a stiff lever!

If that doesnt work i will just rebuild my own calipers and put them in as I dont know what else it could be... The rear brake works fine.. the front brake grabs fine but just the lever has too much play in it.

Thanks.
Lack of real pressure on mine before I did this. I knew the brakes were excellent as I took the whole system off my other bike I had ridden. No real feel on the lever but worked ok.
 

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Discussion Starter · #8 ·
Lack of real pressure on mine before I did this. I knew the brakes were excellent as I took the whole system off my other bike I had ridden. No real feel on the lever but worked ok.
Just tried pushing them out with pads out and lever pumping....they come out unevenly and also i can push one in using my fingers but the other 3 wont budge even trying to push them in with a pry bar!!!!! Any suggestions?
 

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To be working properly they should all move with your fingers ( back in ). When I do it, I work on one piston at a time. Piece of wood in between two opposing pistons, wedge the third,work on the forth and so on. Slow process.
 

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Discussion Starter · #10 ·
To be working properly they should all move with your fingers ( back in ). When I do it, I work on one piston at a time. Piece of wood in between two opposing pistons, wedge the third,work on the forth and so on. Slow process.
I have tried it but no chance getting them in... Tried fingers.... Tried prybar and screwdrivers... Put a old pad on the opposing side and levered with a flat screwdriver and prybar but they dont budge. Only one of them moved freely! Also when i try press brake lever, i see the piston move out but ones i let go of lever, i can see the pistons go back in to its bore just a tiny bit.. 3 of them do that and only 1 of the pistons comes out nicely without going back in... I think I was sold some dodgy calipers?

I guess I will know ones my rebuild kit arrives and I rebuild my old calipers.
 

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Your having a go the right way. Best way forward now is force the pistons out one at a time. Carefully take out the dust seal & clean behind the seal, red rubber grease the seal\ polish the piston when ready to refit ( leave the fluid seal in place && untouched ). Again painful process because you have to rebleed each time, and check the piston is moving freely before moving onto the next piston.
 

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It`s the dust that dirt & corrosion gets behind, causing the dust seal tight onto the piston, restricting the movement. So the cleaner the better.
 

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Buying used calipers to solve sticky pistons in your originals is never a good idea, especially in an environment like the UK.
Rebuild the calipers with new seals as has been advised. Make sure the pistons are immaculate.
Be very careful, the pistons are a very soft alloy and if you grab them with any kind of tool to get them out you will ruin them.
Also make sure the grooves where the seals go in the calipers are totally clean and free of corrosion, but scrape only with plastic or wooden tools.
I had a problem bleeding when I did mine recently and it turned out that I hadn't tightened the banjo bolts on the lines enough and air was getting in, so watch the torque settings.
 

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why it's a good idea to clean up your calipers and such routinely, warm soapy water in a spray bottle works OK, brake cleaner can remove paint real fast so... rinse soapy water off with clean water.. I have a HD spray bottle creates a pretty strong stream, knocks stuff away well, works on chain area too like counter sprocket build up, go very heavy on the Dawn dish soap, soak area, let sit, enzymes will loosen most lubes... then knock away with stream, some older clumps I might have to knock away with a chopstick but rinses pretty clean with spray bottle, cleans up rear as well, once soft I wipe off with rag... I don't like a dirty bike
 

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if this is a jack I can move it somewhere, is there maintenance forum? tech forum?

also for anyone interested in a nice clean look around the chain I use the Moose racing chain brush with the Maxima chain wax care kit, the wax is a good lube and not flingy,, they sell 2 kits, one with chain wax for street bikes, one with chain guard for dirt bikes..

it's a good system, you get a chain cleaner, like the dish soap you spray on let sit, run chain through moose brush then spray off with pressure hose and compressed air if you have it, gets chian good and clean with minimal work, dry bike then spray with MPPL, wipe clean. intermediate lube that pushes any left over water out and seals everything..

then lube with wax, or your favorite I think y can buy separately? good stuff for guy who just like to clean lube and go without messing rags and such...

 
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