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Discussion Starter · #1 ·
first time posting so here goes..front brakes on my '02 sprint ST. When i picked the bike up the dealer said brakes were fine (oh good!!) but there was a lot of travel. Indeed they were fine but with a good squeeze the lever will touch the throttle, i've done all the usual, changed caliper seals, changed M/C seals, tied the lever to the bar overnight, all to no effect. I know people have had success with 5/8" M/C and even running two lines from the M/C and doing away with the crossover hose.
However having stripped the M/C it occurred to me a possible root cause..... both seals are orientated the same way, with the "cup" of the seals towards the outlet, if a small vacuum is generated when the lever is released, as happens to withdraw the caliper pistons surely this could draw fluid from the back of the seal??
To my mind this could craw air in past the outer seal and subsequently draw air/fluid past the inner seal.
Anyone else find this design a bit odd, or is it me???
more to the point has anyone ever really solved the spongy lever travel issue....
 

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This topic has been beaten to death both here and on other lists and other forums here like the S3 one. I have an upgraded 5/8 mc, dual kinda equal length SS lines, improved pads and teflon pistons. The brakes are rock solid, but I could get them just about although not quite by exercising and cleaning the pistons whenever I felt the travel getting a bit long. Every 3 to 6 months depending. So far the mods haven't required that routine but I haven't been riding it as much as normal either.

I have an 02 ST. IMO the situation with the 1050 bikes has similar symptoms but not the same so be careful on comparing the issues. Also IMO you very well may have an excellent point on the MC piston alignment. Definitely worth checking.
 

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Although I'm probably goin to catch hell from many of you Sprint guys/gals here, but, this finding rather supports my argument all along; namely, the Master Cylinder is the culprit.

I've since bought a newer used ZX10R M/C, which to my dismay is a 14M, but has worked flawlessly!

Although I will need to put several more thousand miles on before writing my findings, in the short term, I'm very pleased.See This Link for a few responses.
 
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Discussion Starter · #5 ·
interesting comments.... i had an 04 ST when i lived in the US which was fine, but never checked if it had the upgraged 5/8" M/C... to my mind the whole issue is around correct bleeding of the lines.... the primary seal is in effect the one way valve in the system the (threaded) area between seals is the void that the system draws fluid from when the lever is released, if any air is left in this void then it can and will be drawn into the lines when the lever is cycled.... the swept volume of the 14mm M/C is not far off the total volume of fluid required to move all 8 pistons a few mm!! . by going to 5/8" the increase in swept volume is almost 30%.... well worth it i think.
thanks for the comments guys, i think the original design is marginal, and the 5/8" ID M/C will certainly help, but correct bleeding and especially vacuum bleeding will draw air from the area between the seals into the lines which can then be purged through as normal........ who would have thought that stopping it could be so much fun.. :wink:
 
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Discussion Starter · #6 ·
just to follow up.... having bled the system with all the new seals i cable tied the brake lever hard to the throttle for about 24 hours, any air trapped betwe seals then had the chance to migrate into the M/C RESULT!! the lever has a much more solid feel and cannot be compressed to the bars!! lets see how long it lasts then..... :razz:
 

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Just to make absolutely sure we're on the same page, we are talking 955 Sprints. I've heard the air in the line business now for three plus years and it doen't wash. I've got speed bleeders on each caliper, one at the banjo bolt at the MC, and have bled this system every way known to man. I have discussed this issue with guys who have used various types of power and vacuum bleeders. Bleeding is NOT the problem. There is no doubt that some individual bikes do have a problem because of improper bleeding but the general issue is NOT air in the line from improper bleeding. This air in the line business has caused people to remove the cross over line and to replace by individual lines - doesn't work. Guys have pulled the calipers and hung them in a vertical chain to allow air to bubble up - doesn't work. Air "may" be getting into the lines BUT it is happening AFTER they have been bled properly. If the system is a closed system, which it is, then why can you make the problem go away for long periods of time by exercising the pistons WITHOUT having to bleed or add fluid????
 
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Discussion Starter · #8 ·
I know others have similar problems but, yes, mine is the 955 Sprint and from my point anyway having had a good long look at the design of the system, the M/C has two seals, primary seal to activate the caliper pistons and the back up or wiper seal. The orientation of the primary seal means it acts as the non-return valve so when compressed it passes the inlet port and compresses fluid, when retracted fluid can be drawn from the void between the two seals as the partial vacuum is generated as the fluid retracts the caliper pistons.
In my case what i think was happenning was air was trapped in that area and being drawn into the "live" system on every stroke. This may well self bleed that area but then the air must be bled from the system.
There are some obvious traps in the system such as the top of the crossover line, so i also had the trailing caliper up in the air overnight and bled that one in a vertical position. I may have been lucky in bleeding in the right order, but the behaviour of the system was EXACTLY like having air in the line.
The other thing to note is the best way to vacuum bleed would be with the brake lever tied to the bar so fluid is now being drawn into the area between the seals and over the lip of the primary seal to clear the air out.
This may well boil down to a procedure deal with things being done in the right order, i still think the Ø5/8" M/C would help as the extra 30% fluid would mean that full compression is reached sooner.

Hope that helps to clear up what worked for me.
 

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On 2007-01-09 08:45, jwhitehead wrote:
interesting comments.... i had an 04 ST when i lived in the US which was fine, but never checked if it had the upgraged 5/8" M/C... to my mind the whole issue is around correct bleeding of the lines.... the primary seal is in effect the one way valve in the system the (threaded) area between seals is the void that the system draws fluid from when the lever is released, if any air is left in this void then it can and will be drawn into the lines when the lever is cycled.... the swept volume of the 14mm M/C is not far off the total volume of fluid required to move all 8 pistons a few mm!! . by going to 5/8" the increase in swept volume is almost 30%.... well worth it i think.
thanks for the comments guys, i think the original design is marginal, and the 5/8" ID M/C will certainly help, but correct bleeding and especially vacuum bleeding will draw air from the area between the seals into the lines which can then be purged through as normal........ who would have thought that stopping it could be so much fun.. :wink:
Interesting jwhitehead,

I have an 04 Sprint and I do not have a problem either, mine only get slightly spongey after a while but never serious travel.

But then I change tyres rather frequently and I wonder now if that is why I havent had a problem.

DaveM :cool:
 
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