New disc brakes, new problems

I just replaced the front brake discs on my '90 740 Turbo, as the old
ones were warped and worn down. I thought the pads would adjust to
the new, thicker discs, but a test drive after the reinstall shows
that the calipers are binding on the new discs. Should I use a c-
clamp to depress the pistons, or bleed the brakes to relieve the
pressure on the new discs? Any advice much appreciated.

 

In an earlier contribution to this discussion,

I'm not quite sure what you're saying is happening. Are you saying that
there's metel to metal contact between calliper and disc - or simply that
the pads are binding and not releasing properly after you've applied the
brakes?

Do the callipers each have two pistons - one per side - or is there only
one, and a mechanism which allows the calliper to move sideways to balance
pad wear? If the latter, I would suggest that you may not have re-fitted the
callipers correctly, thus preventing this sliding from taking place.

Whatever the cause, I can't see how bleeding the brakes would help.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

 

No metal to metal contact, the brake pads are not significantly worn.
There is one piston per caliper. I believe I refitted the calipers
properly, its just that the new discs are obviously thicker than the
old, worn ones. In manuals I have, the c-clamp method method of
depressing the pistons is used when replacing brake pads, but there's
no mention of this when replacing the discs. Should I try this, or is
there another solution? Thanks for the advice.

 

Use the "C" clamp to depress the caliper piston.

Or, as I did, go buy a tool specifically designed for this purpose.

 

Strange question.
If he's competent to replace the callipers, I can't see why remembering to
refit the pads would present significant difficulties.

- or

 

If the pistons aren't retracting normally, you've either refitted the pads
wrongly or the pistons themselves are starting to seize.
Difficult to imagine that if they were working normally before, they'd
suddenly seize up - which only leaves a correct calliper/correct fitment
issue.

 

Disks drag. That's a fact of life.
The only reason you'd need to use the C clamp to push the pistons back
is if you couldn't get the calipers over the rotors... which obviously
wasn't a problem.

If the rotors were 'warped and worn down' then the pucks would have
groves and stuff matching the old rotors. Not the best situation for
good stopping with new rotors.
They may wear in over time, or you could pull the pucks and file them
flat, or you could put new ones in and do the job right.

One has to wonder what the cost savings vs extra labor to not put new
pucks in the same time as the new rotors..?

 

Hi Leftie,

I think your contribution may have "hit the nail on the head".
I recently experienced this failure to get the piston to retract all the
way, in order to insert new pads. It was on a Honda motor cycle, not our
Volvo.
Crud and corrosion had accumulated near the outermost portion of the piston
due to its long exposure to the elements with worn pads. (When the pads are
worn the piston remains further out to maintain near contact with the
thinner worn pads.) Instead of replacing the caliper I was able to remove
this stuff by rubbing the piston carefully with very fine steel wool and
cleaning with "brake clean". The piston was then pushed in all the way with
no trouble, the new pads inserted, and the brake has worked perfectly since.
No leakage.

Andy I.


: Pistons on calipers often get 'habituated' to a certain extension,
: and don't want to retract enough to install brand new pads (or rotors).
: This is usually because some rust has formed on the slides and/or
: pistons, near the tops. I remember thinking I had the wrong pads for my
: 122S back in the day, because of this. If you can't get the pistons to
: retract with some careful application of force and slide lubricant, you
: need to replace the calipers.
:

 

Pistons on calipers often get 'habituated' to a certain extension,
and don't want to retract enough to install brand new pads (or rotors).
This is usually because some rust has formed on the slides and/or
pistons, near the tops. I remember thinking I had the wrong pads for my
122S back in the day, because of this. If you can't get the pistons to
retract with some careful application of force and slide lubricant, you
need to replace the calipers.

 

I should have mentioned the option of cleaning the pistons, but I
thought it unlikely that you could get them smooth enough to not damage
the seals as they retracted. Looks like I was wrong!

 

In an earlier contribution to this discussion,

But aren't you missing the point that the OP has actually re-installed the
pads, having fitted the new discs - so he must *already* have retracted the
piston sufficiently to get the pads in.

Since this is a single-piston calliper, the calliper has to be able to slide
sidways in order to equalise the pressure on the two pads. There is much
more force available to initiate this sliding when the brakes are being
applied than there is when they are being released. My money is on partial
seizure of this sliding mechanism (or incorrect assembly) - which is
continuing to hold one of the pads against the disc after the brake pedal
has been released.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

 

What ever happened to a smooth finish with fine emery paper to remove any
ridges

 

Haven't ever done it to a Volvo, but I've maintained almost 40 years
worth of front disk brakes on VW vans.

There is no need to live with brake drag on these or any other
similarly designed disk brake.

With them it was most important to clean the crud off the insides
bores of the cylinders, and also make sure the piston (not "pucks")
sides were smooth and free of rust, scale, scratches and scoring.

But the most important part was the seal. It gets hard and inflexible
with age, and especially heat, and must be replaced. The seal
actually is the part that pulls the piston back ever so slightly into
the cylinder when the pressure is realeased, thereby allowing the
brake pad to disengage with the disk.

I have actually on occasion dismantled everything just to change the
seals after a trip in the mountains when I know I have cooked the
front brakes a bit too much.

Doubles or triples the life of the pads.

 

I was obviously able to get the calipers on the disc, but a mile long
test drive made one of the wheels emit a tiny amount of smoke. I
believe that pushing the pistons back just a smidge should solve the
problem. The brakes had plenty of stopping power before with no
overheating problems, so I'm pretty sure the piston seals are still
fine. Or at least I hope so. My experience of rebuilding the master
cylinder, bleeding brakes and replacing the piston on my motorcycle
makes me never want to relive the experience, much less on a car.

 

In an earlier contribution to this discussion,

Why?

As soon as you apply the brakes again, the piston will be back to where it
is now. You need to solve the problem of why it isn't retracting on its own.

Which pad is fouling - it is the one on the piston side of the calliper, or
the one on the 'solid' side?
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

 

A little smoke could just be the oil burning off they pickle the disks
with so they don't rust on the shelf.
If the pistons are sticking and truly dragging, pushing them back won't
help and will probably hurt.
Soon as you step on the pedal, they'll pop back and stick again.
Pushing them back pushes them over the spooze thats accumulated on the
bores behind the pistons and that can lead to more sticking and leaking.
Or, they could free up, not leak or stick again, and you're good to go...

 

My money's on the slide mechanism.

If the reason for changing the warped discs was shimmying on braking, then
it's almost guaranteed that the slide pins are worn out - allowing excessive
flotation.

New slide time (the part that clamps the non-piston-side pad to the disc and
runs on two pins).

 

The disposable consumer culture.

 

Speaking of 'warped' rotors... The brakes on my '83 245 pulsed badly
under braking. I could feel the warpedness in the pedal and wheel.
When the master cylinder went out I elected to change the pads all
around since I would be visiting each corner to bleed the brakes anyway.
Knocked the glaze off the rotors with some 80 grit emery (the heck with
fine *g*.)
Braking was smooth as silk after that. No pulsing at all a year later.
Sometimes warped isn't really warped...

 

The one thing I can say is that the new discs have eliminated the
brake shimmy, so clearly the old discs were the source of the problem.
I could feel the wheel grab at precise intervals when I rotated it
while the car was jacked up. With the new discs the wheel grabs
consistently and will not rotate freely.