Subject: Re: [Mgs] Brake Caliper Problem-Thanks

From: David Ambrose <[email-masked]>
Date: Wed, 30 Jul 2014 14:58:43 -0700

In my experience, brake rotors almost always have some wedge. If you 
have a micrometer, you can measure it. If you can turn the rotor by 
hand, you should be fine. The piston will retract a bit while driving.

On 7/30/2014 5:04 AM, Matthew Milkevitch wrote:
> 4) One additional
> observation:  the brake rotor rubbing is not constant over the total 360 deg
> rotation.  It turns freely during a part of the rotation, then another part it
> drags.  I'm therefore wondering if perhaps the rotor is warped or the mating
> surfaces between the rotor and hub have some junk in them.  I will disassemble
> that one side today and see what's going on.
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From: Mark <[email-masked]>
Date: Wed, 30 Jul 2014 17:26:41 -0500

I am a little late to this post but it is my understanding that the 
slight "wobble" on disc brake rotors is what knocks the pads back from 
the rotor and that if you can rotate a hub by hand you are good to go.  
Drum brakes rely on springs to move the shoes back away from the drum, 
disc brakes don't have them.  Some of the better pads are ground at an 
angle on the leading edge to make this easier to accomplish, actually 
they are ground on the leading and trailing edge so it won't matter 
which side they are installed.  I agree with David, go drive it around 
the neighborhood for a while.  I have seen both calipers lock on the 
drum if the one way valve in the master sticks closed, in that case 
usually letting a little fluid bleed off will release the pads.  It 
seems to happen more often if you vacuum bleed the system but not sure why.

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From: Richard Ewald <[email-masked]>
Date: Wed, 30 Jul 2014 16:07:18 -0700

Actually the "retraction" of the pads is mostly in the caliper seal. When you
step on the brakes the piston is pressed forward a couple of thousandths of an
inch so the pads press against the rotor. This minor movement is not the
piston moving in relation to the seal, but rather the piston seal distorting
just a bit to allow this very minor movement.
Take you foot off the brake and the seal returns back to normal.  Of course as
the pads wear the piston does have to move outward.
As far as the chamfer on the pads go that is for noise prevention. Back in the
days of square cut pads when we had a noise complaint we would chamfer the
leading edge to make them quiet. Finally the pads makers started doing that
from the get go

A normal disc brake will have a bit of drag when rotated and it won't be even
all the way around. Expect a scritch-scritch sound maybe  1X per revolution.
This is stone normal. If the other wheel has no sound when turning that wheel
might be the issue perhaps a loose wheel bearing or warped rotor
Rick

Sent from my iPhone

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From: "PaulHunt73" <[email-masked]>
Date: Thu, 31 Jul 2014 08:12:45 +0100

It's a combination of the two things.  If you press the brake pedal at a 
standstill, let go, and immediately turn the wheel, there will be some 
residual friction from the pad that rapidly eases, usually leaving just a 
gentle rub.  When driving the bearings have .002 to .004 thou end float, 
which means the hub and hence the disc can 'wobble' a bit relative to the 
caliper, and this tends to knock the pads back a little bit more.

But did the original poster ever come back with anything, I've not seen it, 
but then I didn't get the original post either, just MGBob's response.  As I 
said before the first thing to do with a problem like this is manually 
retract the pistons and see how the wheel turns then.

PaulH.

----- Original Message ----- 
> Actually the "retraction" of the pads is mostly in the caliper seal. 

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