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What Have I Done To My Brembo P4's?


Bob Hartman

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2002 Le Mans with 23K miles.  While the front wheel was off for a new tire I decided to clean the calipers.  Still connected, pads in, I brought a bucket of water with a little dishwashing liquid up to them and scrubbed with a nylon bristled dish brush.  No drama. How much water would they get during a ride in a rainstorm, right?

 

Once reinstalled they pumped right up but as soon as the bike moved the brake lever went to the bar.  I bled them, same thing.  Bungied the lever back overnight, same thing.  Bled the M/C, same thing.  The lever gets pressure but loses it the moment the wheel turns.

 

So - removed the calipers, emptied the system, removed the pucks one at a time - all looked and felt fine, although a couple had feint striations of lighter color I couldn't feel.  Buckets were perfectly clean.  Seals felt fine to my finger and didn't show any visible damage.  Reassembled everything using brake fluid as lube.  Same thing:  Lever pumps up and holds until the bike moves.  

 

The odd symptom is the lever pumps up to perfect - and stays like that - until the wheel goes a few rotations.  Then all bets are off.

 

I've done loads of brakes in decades if riding without problem so I can't call this one.  Anyone know what's going on?

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The odd symptom is the lever pumps up to perfect - and stays like that - until the wheel goes a few rotations.  Then all bets are off.

 

 

 

Can you try that with the front wheel off the ground? If the lever gets pressure again, see if the wheel will turn while the lever has pressure. If so, you probably have a blocked line.

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The odd symptom is the lever pumps up to perfect - and stays like that - until the wheel goes a few rotations.  Then all bets are off.

 

 

 

Can you try that with the front wheel off the ground? If the lever gets pressure again, see if the wheel will turn while the lever has pressure. If so, you probably have a blocked line.

I did try that.  The wheel is immovable while off the ground and lever has pressure.  Release lever, rotate wheel and pressure drops.  No leaks anywhere, should have said.

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And the brakes were working perfectly before this?

 

Like Thumper says, it's probably worth trying again to get rid of any possible air.

 

After that, I dunno... weird that it holds pressure, then doesn't.

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Have you got the front wheel spacer in/in correctly? Has your friendly tire fitter bent a front disk when you've had the tire replaced?

 

IE...... fit the wheel and good lever, rotate the wheel and the bent disk pushes the pads back and.....no lever. 

Ciao

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A few years ago, my Suzuki TL1000 S developed the same problem.  I hadn't done anything to the brake system beforehand - it just appeared one day.   Brakes were tight and normal at a stop but almost immediately the lever went to the grip when moving.  I could  pump the brake lever up some but it immediately goes soft again.  At a stop, everything is okay.  I bled the system numerous times but the problem persisted.   I've been tinkering with motorcycles for over 40 years and I was really stumped.  The bike was basically unrideable because of this problem.  One day when testing it in my neighborhood, I watched the brake discs as I was moving slow and noticed they were warped.  I replaced them and the problem went away.

Maybe this will help.

 

Frank

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A few years ago, my Suzuki TL1000 S developed the same problem.  I hadn't done anything to the brake system beforehand - it just appeared one day.   Brakes were tight and normal at a stop but almost immediately the lever went to the grip when moving.  I could  pump the brake lever up some but it immediately goes soft again.  At a stop, everything is okay.  I bled the system numerous times but the problem persisted.   I've been tinkering with motorcycles for over 40 years and I was really stumped.  The bike was basically unrideable because of this problem.  One day when testing it in my neighborhood, I watched the brake discs as I was moving slow and noticed they were warped.  I replaced them and the problem went away.

Maybe this will help.

 

Frank

Trashed wheel bearings will do the same but I'm under the impression here that if you push the bike a few wheel rotations it will happen, so a bent disk is probable. Worn out wheel bearings need to be ridden before they do the same thing.

 

Ciao

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Yeah, it certainly *acts* like a bent disc, but that should have shown up before the cleaning job. For sure, I'd put an indicator on the disc and rotate it. Any more than .002" runout is a problem.

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A warped rotor would just pulse the lever when braking, not collapse it. Unless WAY out, which you would have noticed before.

 

Well, if the rotor IS warped, and you bled the brakes when the rotor was at/near it's closest point to the caliper, than it could collapse the lever when it rotated and wobbled away from the caliper.

 

If the rotor is warped, try bleeding when the warp is furthest from the caliper and see if that works. If so, than it indicates the rotor, not the hydraulics.

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  • 2 weeks later...

Apologies for letting this go; other stuff kept me out of the garage til now.

 

 

The answer, of course, was a disc.

The mystery was everything worked perfectly, the wheel came off for a new tire (installed by me on the Harbor Freight gizmo), the calipers got cleaned in situ, the wheel went back on and the brakes developed issues.

No other symptoms, noise, vibration, etc., So the focus was on the brakes and hydraulics.

 

Turns out that one of the discs evidently got leaned on, the outer disc shifted and a floater button or two were too tight to let it return to position.

Rotation of the wheel revealed a slight bulge not immediately visible to the naked eye.

Application of cleaning lubricant and the rubber mallet got everything to let go and return to shape.

 

Thanks to those who replied, much appreciated!

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there was a thread some time ago regarding those rotor buttons. oxidation can make them stuck. remedy is to make a bolt/nut set up to clamp the button, and spin it with a drill. It won't work on my '00 but will on the '02 which has through holes.

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My '02 Le Mans doesn't happen to have holes in the buttons, but not a worry now that I'm aware they need attention.  Never occurred to me I'd done anything to shift the disc.  

 

The misleading surprise was that while riding some diagnostic miles there was no pulsing or any indication anything was amiss other than the handle going to 10% before applying brakes. 

 

I've had bikes with floating discs that rattled at idle. That may not be the norm here, but might be a good thing. 

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