Inconsistent squishy brake??

I just took my 2007 ZX-6R out of my house today after hibernating in my dining room the last 4.5 months. Started right up (though it was on a battery tender). My front brake was soft but it pumped up firm right away. Then I drove it down the street and when I applied the front brake it again was very soft but...it pumped up firm right away. This was a consistent pattern - go soft but pump right up firm. The resevoir is full of brake fluid.

I am at a loss for an explanation on this one ...any mechanical slueths out there who can pinpoint this?

By the way, last November when it was last ridden no such issue existed.

Thanks,
Steve

 

Bent rotor or rotors possibly.

 

change the brake fluid.

 

Flush and bleed. If that doesn't work after a couple of bleeds, than I would inspect the entire brake system.

 

+1 for possibly bent rotor.

 

I suspect the same.

Easy way: get the front wheel off the ground and spin it while observing either brake rotor.

Better way: remove front wheel and check runout on a wheel balance tool.

 

Bleed the master cylinder first, couple of pumps on the brake and crack it open, close it. could be air in cylinder. air rises to the top first, think of it that way...then bleed the calipers and you should be fine.:up:

 

Bent rotor


Each time the bent part of the rotor passes through the calipers it pushes the pistons back away from it. So now you've got a dead space between the piston/pad/rotor. Just like when you take a caliper off and spread the pads apart, when you put it back on the lever feels soft and you have to pump it a few times to push the pads back up against the rotors. Same thing with a bent rotor.


Make sense?

 

How could he have a bent rotor from the bike sitting over the winter?

 

Something could have fallen on the rotor or bumped into it and bent it, or it was bent before it was stored.

 

Accidentally kicking it walking past, moving things around in his living room and bumping it, kids hitting it with a loose soccer ball...

Who knows?

 

Put some castrol SRF in it and bleed them properly. :up:

 

A bent rotor he would notice via vibration. My money's on bad fluid.

 

Not based on the symptoms he's describing.

A very slight warpage could be felt by pulsing at the lever.

But pumping the lever solid, going right down the street and the first time he grabs the brakes they are soft again is not going to happen from unbled fluid. If there was air in the lines, it wouldn't pump solid in the first place

 

I vote bent rotor. I have the same bike and I didnt bleed the system for 3 years, never once had spongy brakes, even after sitting all winter. Obviously check the cheap stuff first though.

 

A small amount of air in fluid that has picked up water results in very varying brake values. If you want to figure it out, first get rid of the old fluid. You can put the bike up on a front stand, spin the wheel and look from the front and SEE warpage.

 

I had one act like that because of a loose axle bolt. Go around a corner and the wheel shifts just enough to move the pad over and low lever on the next pull.

This thread raises a question for me: what constitutes bent? I had a seized caliper which got the rotor very hot. I checked it and there's a variation of 9 thou. Does that matter? I'm inclined to say it's okay.

 

Full floating rotors are designed to accomodate for a slight bit of side to side play.

 

I race in V7 with old-fashioned bolt-ons. Still seems insignificant, though.

 

A distinct possibility. The last ride back in November of 2010 the brakes were fine. I have had warped rotors on another bike before (Gixxer 750) and the feeling I am getting on the Kawi involves no pulsing or vibrating at all.

If the worst-case develops, what do people recommend for a decent (not expensive top-of-the-line) rotors? I have done some looking (just in case) and there are a lot of choices spread over a wide price-range.