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I was out on the track the other day with my 07 1098s and the brake lever went soft. I have the stock braking system with Ducabike levers fitted. I have tried moving the fulcrum point on the levers but that doesn't help. I have also tried bleeding the brakes several times and leaving the lever zip tied to the bar over night. The brakes will feel good for a minute and then go soft again.

I know this was a problem in the calipers on the early ZX10r, but has anyone heard of this on our bikes?
 

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'12 848evo, '13 Hypermotard SP, '94 CBR600f2 track bike, '06 CBR600rr track bike
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I would hold pressure on the lever and see if it comes back to the bar after a bit, could be worn master cylinder seals.
 

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Had similar issues on my 999s track bike. Couple possibilities.

a) the brake fluid is laden with moisture. all is well until the caliper and the fluid within gets hot enough causing the water to vaporize creating bubbles. If things seem to return to normal after everything cools down suspect this.

b) air is entering somewhere, only two places this can happen.
1) down at the caliper pistons master pistons. Sticky caliper pistons could theoretically lead to air being drawn in around the worn seals but probably not likely.
2) up at the master piston. If the airspace above the reservoir diaphragm is not venting to atmosphere correctly, as the system cools air can to be drawn in around the master piston seal as the system cools.

My problem was 2), first session out everything was fine but after a couple sessions things got scary as the lever began to hit my knuckles under hard braking. Moving the lever out did not solve the problem as it got progressively worse and I found my fingers cramping trying to reach for the extended lever. When I bled them between sessions tons of air would come immediately out indicating the air was entering around the master piston and not being forced from the caliper. I reworked the reservoir cover to insure the air space above the diaphragm was properly venting and that seem to solve the problem. Stock 999's & 749's have die cast reservoir covers with vent grooves on the sealing surface, the rubber diaphragm tends creep or swell into the vent grooves over time. If things get progressively worse AND there is air in the system suspect this.
 
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