Front Brake Gets Spongy


5 replies to this topic
  • roylo

Posted September 27, 2009 - 06:39 AM

#1

My front brake on my 08 YZ450 always looses some of its 'compression' while riding. Sometimes the brake pressure will start to come on near the beginning of pulling the lever and other times it wont come on until its almost hitting my knuckles.
The actuation point could change from corner to corner. If I pump the lever it hardens up and all is good, but a few corners later the actuation point is too low again.
Ive bled the brakes many times and it fixes the problem for a while but it soon returns.
Two things which I think could be happening is air bubbles are getting in the line from the top of the master cylinder reservoir while riding, or, bits of dust/dirt getting inbetween the pads and disk and slightly pushing the caliper apart? The disc does not looked to be warped at all.
I dont know but does anyone else experience this?

  • grayracer513

Posted September 27, 2009 - 07:26 AM

#2

You need to differentiate between two separate conditions.

In the first, the brake pads get pushed back away from the rotor by the rotor having excessive run out, or because the caliper is mounted out of square due to part of the mounting hardware being bent. This condition results in excessive free travel at the lever.

The second condition is one in which air enters the hydraulic system, usually at the master cylinder bore seals, if there is no sign of a fluid leak.

The difference between these two is that in the first case, the brake lever simply travels too far, and if the lever is cycled a time or two, the lever has a more normal feel, being fairly solid as the brake is engaged. If the brake is used frequently, it remains more or less normal, but if not employed for a while, the excess travel returns.

With air getting in the system, the brake develops no real excess lever free travel, but will have a distinctly spongy or springy feel rather than feeling as if the lever has stopped against something solid, and in a bad case, can be pulled to the grip. "Pumping it up" may improve the feel temporarily, but the condition returns almost as soon as the lever is completely released, and gets progressively worse over time.

  • roylo

Posted September 27, 2009 - 09:27 AM

#3

You need to differentiate between two separate conditions.



the brake lever simply travels too far, and if the lever is cycled a time or two, the lever has a more normal feel, being fairly solid as the brake is engaged. If the brake is used frequently, it remains more or less normal, but if not employed for a while, the excess travel returns.

.




Thats exactly what happens Gray. From what you explained my rotor or caliper must be out slightly. Although, the brake does work smoothly and the rotor 'looks' normal. Can dirt/dust or tiny rocks getting inbetween the two cause this aswell?

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  • grayracer513

Posted September 27, 2009 - 12:46 PM

#4

Can dirt/dust or tiny rocks getting inbetween the two cause this aswell?

Probably not. Disc brakes are intentionally tolerant of environmental dirt.

If you have a good feel for how long this takes to happen, ride the bike for as long as it takes to set the problem up, stop without using the front brake, dismount, and watch the pads as you or a partner operates the brake. If the pads move a significant amount the first one or two times the lever is pulled, they are being pushed back by something.

Look down on the caliper toward the axle and watch the caliper as the brake is applied. If the caliper twists or rolls when force is applied, it's out of alignment with the plane of the rotor.

Other causes are rotor run out (measure it against the specs in the manual), and loose wheel bearings.

  • ZéPovinho

Posted September 27, 2009 - 03:34 PM

#5

I had the same thing on my WR, during a race. If I braked and made a corner and headed straight in to another braking/turn, it felt OK. but if I would make a left/right/left section of the track, on the next bracking the lever would go down to the grip. I bleed the brakes 2 times that day. by the end of it still dint know what it was. turned out one of the front weel bearings became a bit loose in its casing from a tight axel on a tire changer I owned, that made the bearing pop out from once in a while, to almost always.
I dont imagine you change tires that often but you know that the japs at the factory are not very kind on grease so you should check if your wheel moves from side to side when assembled to check if your bearings are OK.

  • roylo

Posted September 28, 2009 - 01:57 AM

#6

Took the front wheel off and checked the whole thing out. There was some old grass built up between the pads and the caliper which couldnt be seen with the wheel in place. Cleaned it out and lightly greased the caliper sliding pins, went for a ride seems ok now. Thanks for the help.





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