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Discussion Starter #1
I have an '05 999s, less than a year old, bought new, no accidents, with 10000kms on it. I have always used fresh dot 4 fluid. There are no leaks.

I have had to bleed the front brake twice since I bought it due to 'spongy lever feel'. In both cases the lever got progressively 'spongier' over time. Once bled, the lever feel was fine for a couple of months until slowly degrading back to spongy again.

The last time I bled the brake, I put a bit more than normal fluid back into the resevoir because I was thinking that it was possible that cornering was uncovering the master cylinder fluid inlet and allowing air in, (I often trail the front brake into corners).

Today, went to the track. I only bled the front brake about 6 weeks ago. Brake feel was fine riding to the track. After the first session, bit spongy. After the second, more spongy. During the third session, my lever was coming all the way back to the bar, so my day at the track was at an end.

The fact that after I have bled my front brake in the past, my brake lever returns to normal says to me that I'm getting the air out of the system, (even though I don't see bubbles which I find a bit strange).

Is my master cylinder cactus?
 

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Brake bleeding

There is a 6mm bolt head inside the master cylinder that can be seen when you pull the cover off. Back this bolt out slightly while squeazing the brake lever and bleed the master cylinder. Tighten it just like you would with the caliper bleed nipple before you release the lever. Your problem should be solved. Same thing with the clutch master cylinder.
 

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I also recommend you --COMPLETELY-- flush the old brake fluid. Brake fluid is hygroscopic and absorbs moisture. That moisture after the brake fluid heats up will turn into air. Air is compressible which results in the spongy feel that you are describing.

A 2% moisture content will drop the boiling temperature of your brake fluid by almost 100 degrees F. The following two articles have some very good information about what happens to brake fluid as it gets old. I recommend you read these articles. They might provide some additional insight that might be helpful in your case:

http://www.aa1car.com/library/bfluid.htm

Does Brake fluid wear out?

If you decide to flush your brake fluid, I suggest you remove as much fluid as you can from the reservoir by bleeding the old fluid out before adding the new fluid to it. This will insure that you get most of the old fluid out.

-Fariborz
 

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Discussion Starter #5
OK. New fluid (again). Bled calipers, and have also bled the master cylinder which was where the air was. Brakes are back to normal. I'll see how long it lasts.
 

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Spongy Brakes

I bled the calipers and thanks to you guys...also bled the master cylinder and my brakes on my 749r feels like new again. Thanks for the advice. :D
 

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Love this forum. Just swapped out the stock calipers on my '91 900SS with those from a '98 748, and installed new Galfer braided lines. I'm sponge city. Driving me crazy. Going to try the MC fix today...thanks!
 

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I've had exactly the same problem on my 998. The MC (PS 16) was faulty.
I may be looking at the same thing. I've tried the other options:

  • I zip-tied my front brake lever to the grip and left it for a few days. Still spongy.
  • My MC does not have the small bolt that is described above, so I can't bleed it.
  • After going back in and bleeding the brakes again, I still have sponge.
  • Brake lines are brand new Galfer stainless - brand new crush washers, etc. I don't see any seepage at the banjo bolt linkages...

What happens is that I can squeeze the brake lever all the way to the grip. I can pump it and it gets better, but then I lose pressure. Feels like I'm losing pressure at the MC.

So I'm after a MC for a 1991 900SS. Anybody have any suggestions?
 

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THANKS for the tip. I had spongy brakes after changing to spiegler lines on a 2004 999. Bled the mastercylinder screw inside the reservoir. Immediately fixed it. I Love this forum.
 
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