New disc brakes, new problems

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.

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

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

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!

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.

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.

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?

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.