Brake Lever Travel

[The Stranger:Me]

Ok, the YZF750, it is just about back on the road, but it still has the old problem with the brakes.

It has 6 pot calipers on the front and when I apply the front brakes there is a massive amount of travel in the lever.

Now it does pull you up and if you hold the lever in with a constant pressure it does not slowly loose pressure and continue travelling in.

I have bled and bled the system and am fairly sure there is no air in there, but it would appear that it is just that there are so many bloody pistons to push that the volume of fluid required is greater and thus additional lever travel is required.

But one thing I have noticed is that the brake pads retract off of the disc quite a way when the lever is released, like it may be less that a mm but then the fluid required to replace the travel is quite a lot.

So.
Is it normal for the pads to visably retract from the disc and does anyone have any ideas as to what else may be causing the excessive lever travel.

[imdying]

Are they the factory calipers, master cylinder and/or hoses?

[sAsLEX]

The distance the pads a re retracting kinda points to the discs being out of true

[Mental Trousers]

Got the same bike (SP model, is your's an R or SP??). The lever travels almost all the way back to the bars, but I'm perfectly fine with that. And she stops (nobody got pics of me pulling stoppies at the end of the straight at Taupo unfortunately ). I actually prefer the longer travel because the 6 pots can be bloody vicious so having more travel in the lever gives finer control.

Mine doesn't seem to withdraw the pads from the disk though.

[The Stranger:Me]

Are they the factory calipers, master cylinder and/or hoses?

They are factory calipers, I am very sure that the master cylinder is original. I have after market braided lines.

[The Stranger:Me]

The distance the pads a re retracting kinda points to the discs being out of true

The discs don't appear to be out of true to the feel, and besides this is at stationary.

[imdying]

If they're working ok (power and predictability) then it comes down to taste. If you'd like less travel, you can put a larger bore master cylinder on there. The trade off will be an increase in required lever pressure.

[The Stranger:Me]

Got the same bike (SP model, is your's an R or SP??). The lever travels almost all the way back to the bars, but I'm perfectly fine with that. And she stops (nobody got pics of me pulling stoppies at the end of the straight at Taupo unfortunately ). I actually prefer the longer travel because the 6 pots can be bloody vicious so having more travel in the lever gives finer control.

Mine doesn't seem to withdraw the pads from the disk though.

Ok that is interesting.
Do the brakes apply progressively during that travel?
Mine the lever travels say 3/4 the way with no braking effect then all effect is in the last 1/4 of travel.

The X11 starts braking in the first few mm of travel and I doubt that the lever has ever been beyond half travel.

[sAsLEX]

The discs don't appear to be out of true to the feel, and besides this is at stationary.

Well the only reason the pads and pistons retract is as they are pushed back in by the disc.

[imdying]

Pop out and look at the master cylinder, what size is cast into the body? It'll probably be either 1/2, 14 of 5/8.

[imdying]

Well the only reason the pads and pistons retract is as they are pushed back in by the disc.

No, the reason the pistons are sealed with square section o-rings is so that they deform (inner side shears out like a parallelogram) when you put the brakes on. Then you release the brakes, the parallelogramed seal reverts back to it's square shape, retracting the pistons.

[The Stranger:Me]

Pop out and look at the master cylinder, what size is cast into the body? It'll probably be either 1/2, 14 of 5/8.

Bike is not handy, but I have seen 5/8 on the body.

[sAsLEX]

No, the reason the pistons are sealed with square section o-rings is so that they deform (inner side shears out like a parallelogram) when you put the brakes on. Then you release the brakes, the parallelogramed seal reverts back to it's square shape, retracting the pistons.

how far though? Always thought the discs played a part.

[imdying]

That's about as large as a your typical bike master cylinder gets, which is unfortunate. Might just have to lump it (or get some r1 calipers). You could of course overbore that cylinder, but it'd depend on the amount of meat in it.

[Mental Trousers]

Ok that is interesting.
Do the brakes apply progressively during that travel?
Mine the lever travels say 3/4 the way with no braking effect then all effect is in the last 1/4 of travel.

The X11 starts braking in the first few mm of travel and I doubt that the lever has ever been beyond half travel.

Because mine don't actually seperate from the disk they come on early and softly, getting extremely strong near the end of the travel. They're so strong the forks bottom out (standard springs soon to be changed) and the front end then starts bouncing like a pogo stick.