pulsatile brakes on my LS

[dorydan]

Hi Guys,

The Brembo brakes on my '82 LS pulsate slightly.  It is only noticeable at very low speed (when coming to a stop) but actually causes the forks to compress/decompress slightly ,once with each wheel revolution.  The bike has 14k miles, the rotors are just over 5mm thick, not scored, not rusted, look great, and the runout is 0.002" (R) and 0.003" (L).  The brake lever does NOT pulsate.  So, before I spend a fortune on new rotors, is there something else I should try?  That runout seems pretty low to me, but maybe that amount is significant.

So far, this is what I have done:  installed stainless lines, replaced the fluid, cleaned out calipers, cleaned pistons, and installed new seals.  Sanded the pads, sanded and cleaned the rotors twice.  I've put a few hundred miles on the
bike with no change.  I will say, except for the pulsation, the brakes work great!

Thanks,
Dan

[Bob_Roller]

On the right fork leg, are there hat type spacers in the holes where the bolts that secure the calipers go through ??

[skippyc]

There is one spot on the disc where there is a lot more grip You may find it by turning the wheel by hand with a little pressure on the brake

[dogshome]

Wheel bearings good? Front forks, springs and oil all OK and assembled correctly i.e. bounced on before final tightening?
No bent axle?

I mean, nothing supporting the brakes and wheel is out of line or loose/worn.

[georgesgiralt]

Hello,
I would, if I were on your shoes, remove the brake pads, then rotate the front wheel and see how it goes. Then I would check that the caliper are centered on the rotor. (it's easy to check as the caliper has a line showing the center of it and the rotor is thin...) If they are not centered, you may have some problems when pad wear.
Last, check the pad surface to see if they're flat and not angled versus the support. Clean the rotor with brake cleaner and try the pads back.
Report your findings ....

[dorydan]

Thanks, guys, for all the tips.  The calipers are centered and the little hat spacers are in place on the right side.  The wheel  spins well without pads and the bearings seem fine.  I did release the tension on the axle and pinch bolts, jostled the forks and re-torqued in proper sequence.  Cleaned the disks one more time with brake cleaner.  NO CHANGE!

After dark, I put a bright flashlight behind the calipers and, looking from the front, there is a "slit" of light which comes and goes once per revolution on the left side.  When the light disappears there is a slight audible friction and the wheel does drag very slightly.   

Does anyone know the factory specs on acceptable rotor run-out?  I am thinking I need to shop for new rotors.....

[georgesgiralt]

Before investing for new rotors, remove them and check there is nothing beneath the rotor and the wheel. A little scar is enough. Mark the position of the problem relative to the wheel, remove the rotor, check it and the wheel and put it back. If problem solved, tap yourself on the back. If not, try to change the orientation of the rotor relative to the wheel. Sometimes it is enough.

[dogshome]

If you can see the runout, it's probably too much. 3 thou is 0.08mm, which is a lot now I think about it!

I'm glad you are homing in on the problem 

[dogshome]

Haynes says 0.2mm max runout.

[dogshome]

Hey Dorydan, did you fix it?

[dorydan]

OK guys, thanks for the help.  All great suggestions but to no avail.  Installed new Brembo discs, ferrodo pads, and dang, she brakes smooth as silk and stops on a dime.  Good brakes are nice!