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The Stranger
2nd June 2006, 09:40
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
2nd June 2006, 09:57
Are they the factory calipers, master cylinder and/or hoses?
sAsLEX
2nd June 2006, 10:02
The distance the pads a re retracting kinda points to the discs being out of true
Mental Trousers
2nd June 2006, 10:16
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
2nd June 2006, 10:18
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
2nd June 2006, 10:40
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
2nd June 2006, 10:41
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
2nd June 2006, 10:45
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
2nd June 2006, 11:09
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
2nd June 2006, 11:09
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
2nd June 2006, 11:13
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
2nd June 2006, 11:20
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
2nd June 2006, 11:24
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
2nd June 2006, 11:25
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
2nd June 2006, 11:26
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.
imdying
2nd June 2006, 11:27
how far though? Always thought the discs played a part.The minor (generally less than 2 thou) acceptable runout on discs is responsible for pushing the pads just off of the discs surface, but not responsible for the pistons retracting at all.
sAsLEX
2nd June 2006, 11:47
pushing the pads just off of the discs surface, but not responsible for the pistons retracting at all.

Umm if the pads are being pushed back does that not imply the pistons also are moving back since the two are attached? As I am sure the discs moving at a faster rate than the rubber rings
imdying
2nd June 2006, 12:17
Umm if the pads are being pushed back does that not imply the pistons also are moving back since the two are attached? As I am sure the discs moving at a faster rate than the rubber ringsNo, the pads can only move back because the pistons have retracted. The runout on the discs that pushes the pads off of them is miniscule, especially when compared to the amount of retraction the caliper seals do.
XP@
2nd June 2006, 14:00
how many times have you bled them?
is there a huge air bubble in the calipers?
Did you remove them & all the fluid? if so did the old fluid chrystalize(sp?) in the inside the brakes?
Is the rotor flat?

Being a mechanical spoon I made all of the above errors and at various points had similar effects to what you described.
The Stranger
2nd June 2006, 14:24
how many times have you bled them?
is there a huge air bubble in the calipers?
Did you remove them & all the fluid? if so did the old fluid chrystalize(sp?) in the inside the brakes?
Is the rotor flat?


Well I hope I have bled them corectly and left no air bubbles. I don't think one can ever be 100% certain though.
I had removed all the fluid previously and popped the pistons out and cleaned these up and got all the gunk out from in the cylinders, though there were no crystals.

Last tiem I reasembled them they were the same but yeah just giving them another bash as I had to rework the brakes due to shall we say, an unfortunate incident.

Rotor flat? well not buckled, though there is some ridging and wear, though nothing abnormal. They are a little below the wear limit, but not badly so.
FROSTY
3rd June 2006, 01:02
CAN a couple of points. 1)use rubberlube on the pistons when putting it all back together. 2 I strongly susspect your pistons need to be pumped out another 1/2 mm. Try this trick. Pull one caliper off then give ONE pump on the brakes.The caliper should still fit back over the disk but will be reluctant
bolt it back up and repeat the exersize with the other side.
I bet your brakes will improve heaps
Scouse
3rd June 2006, 01:19
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.Shane dont change yer fork springs change yer bike
The Stranger
3rd June 2006, 15:48
Thank you, will give that a try.
The Stranger
3rd June 2006, 17:46
CAN a couple of points. 1)use rubberlube on the pistons when putting it all back together. 2 I strongly susspect your pistons need to be pumped out another 1/2 mm. Try this trick. Pull one caliper off then give ONE pump on the brakes.The caliper should still fit back over the disk but will be reluctant
bolt it back up and repeat the exersize with the other side.
I bet your brakes will improve heaps

Hmm no real change.
When I pulled the pumped the lever the pistons travelled out ok, but when I released they travelled back the same amount. Tried it with both calipers, same result.

Oh well.
FROSTY
3rd June 2006, 19:53
cam--dude um -it works like this -the caliper seals are big (ish) square section O rings -they act as return springs for your pads.
The way they do that is they distort inwards as you apply the brakes -as soon as you release the pressure they twist back into shape pulling the pistons with them.
What you need to do is get the pistons to move a little bit beyond the return point of the o ring. Then the pads wil be a tiny bit closer to the disks and I bet ya brakes will be tonnes better
Mental Trousers
3rd June 2006, 22:35
Shane dont change yer fork springs change yer bike

Quiet or I'll learn ya.
Scouse
4th June 2006, 00:09
Quiet or I'll learn ya.Oh ok just as long as you don't sin bin me oh mighty Moderator :p
Mental Trousers
4th June 2006, 11:53
Oh ok just as long as you don't sin bin me oh mighty Moderator :p
:motu: Smart arse :finger:
erik
4th June 2006, 19:32
I think I've got the opposite problem, the pistons aren't pulling back far enough from the discs on my bike. Anyway...

I wonder if there might be an air bubble at the top of the lines, near the master cylinder?
Try pushing the pistons back into the calipers so that some of the fluid flows back up into the master cylinder reservoir. I did this on my bike today and it has made the brakes feel a lot less mushy (they were feeling mushy after the bin at taupo, DEATH suggested some air might've got into the top of the lines since the fluid level in the master cylinder was low).
WINJA
4th June 2006, 19:48
The distance the pads a re retracting kinda points to the discs being out of true
MAYBE , BUT CHECK THE AXLE FIRST CAUSE IF THE AXLES BENT THE DISCS SIT ONTHE PISS , HAD THIS WITH THE 9 I DID PADS AND MASTER CYLINDER STILL NO JOY , THEN I CHECKED THE AXLE FOUND IT BENT REPLACED IT AND JOY , A BENT AXLE CAN BE HARD TO SPOT CAUSE ITS POSIBLE THAT JUST THE THREADED PORTION IS BENT ,
GOT A EASY WAY TO CHECK IF YA WANNA KNOW
terbang
4th June 2006, 20:11
Assuming the runout and min thickness on yer discs is ok I cant help but wonder if there is something funny happening with yer master cylinder.
The pads shouldnt retract that much..
The Stranger
4th June 2006, 20:26
Sure Winja. I am listening.
Right now I would try just about anything anyway.
WINJA
4th June 2006, 20:42
Sure Winja. I am listening.
Right now I would try just about anything anyway.
THE 9 HAS A BUSH THE AXLE WINDS INTO UNDO THE PINCH BOLTS SO THAT ITS STILL TIGHT BUT THE AXLE CAN BE SPUN , AS YOU SPIN THE AXLE THE WHEEL SHOULD STAY TRUE BUT IF THE AXLE IS BENT ESPECIALLY AT THE THREADED END WHERE IT WEAKEST THE THE WHEEL WILL WOBBLE BETWEEN THE FORKS, THE YAM MIGHT BE DIFFERENT IF IT HAS A THREAD CUT INTO THE FORK JUST WATCH THE WHEEL SEE IF IT WOBBLES AS YOU LOOSEN AND TIGHTEN THE AXLE, ANOTHER TELL IS ARE YOUR PADS WORN ON AN ANGLE NOT LEADING TO TRAILING BUT THE OTHER WAY
erik
4th June 2006, 23:12
MAYBE , BUT CHECK THE AXLE FIRST CAUSE IF THE AXLES BENT THE DISCS SIT ONTHE PISS...
That reminds me.
On the Zeal the wheel bearings were stuffed letting the wheel move around a bit and when I replaced them, the brakes improved also. So check your wheel bearings as well.

Have you tried Frosty's suggestion except try pumping it twice or three times so that the pistons move out and stay out?
WINJA
4th June 2006, 23:38
That reminds me.
On the Zeal the wheel bearings were stuffed letting the wheel move around a bit and when I replaced them, the brakes improved also. So check your wheel bearings as well.

THATGOES WITHOUT SAYING BUT YOUD PROLLY NOTICE BAD WHEEL BEARINGS B4 IT BECAME A BRAKE LEVER TRAVEL TYPE PROBLEM, AND HOPEFULLY YOUR WARRANT GUY WOULD TELL YOU A BEARING IS BAD B4 THESE ISSUES ARISE .
The Stranger
8th June 2006, 12:45
Ok things I have found out.
1) If you bleed your brakes with the bike on the side stand the RHS caliper will bleed properly, but the LHS one wont. So lean the bike to the opposite side whilst bleeding the LHS caliper.
2) If there is air in the main line and you bleed the brake by pumping the lever the air bubble pushes down on the pump then drifts back up as you release the lever.

Got to admit that number 2 is my theory, so I went and bought a vaccum bleeder and used a vaccum to pull the fluid through and there is a significant difference.

Brakes are still not as good as the X11 but then they are 8yrs and 25,000km older.

I have yet to check the axle, but will shortly. Thank you to all who offered help.
XP@
8th June 2006, 13:02
In addition to the axel check, if you are looking for bends and twists in an older bike then try turning the forks round in the triple clamps, .5mm of a bend can make a huge difference if it is bending the wrong way...
imdying
8th June 2006, 13:36
2) If there is air in the main line and you bleed the brake by pumping the lever the air bubble pushes down on the pump then drifts back up as you release the lever.Yep, you're correct, pain in the backside really.

If you've a bike that simply won't bleed, worse case scenario, you can alway take the master off of the bars and put it lower than the calipers, or vice versa.