brakes squeaking

[dan_b]

Bah, my new front brake setup has now developed a London taxi-esque habit of squeaking/squealing.

There are two different pitches of squeal, one that happens when light brake pressure is applied which is a low-ish pitch, but goes away when you stick more force on, only to be replaced by a very high pitched taxi squeal under harder braking effort. It's annoying as hell!

What causes this and how do I cure it? It's embarassing!


Also, I'm getting an occassional resonance after repeated brake effort, as if the pads don't disengage from the discs fully? Is this normal with Porsche calipers? Anything I can do about that?


Brake force is great though, a real worthwhile upgrade.

[Stick Insect]

Hi Dan,

I can't remember what caliper it is that you use. If it is the split design then the pads could be binding on the pad plates which need to be removed to allow clearance.

if it is the 996 monoblock style then I'm not sure.

The calipers need to be mounted so that the are working on the correct rotation.

Split calipers have a direction of rotation arrow on them. Later monoblocks don't seem to have this arrow but as a simple rule of thumb you may need to check they are mounted correctly.

The smaller piston is the leading piston. You may need to swap the cross over tube over for the bleed nipples. bleed nipples are on the top of course!

If you have the caliper the wrong way round then it may not be moving back as well as it could do.

HTH

David

[dan_b]

Hi David
My calipers are the Porsche 968CS ones, they're black and heavy. They're definitely on the right way round - the Porsche monogram is pointing outwards, the two bleed valves are at the top of the caliper. Re-furbished with all new pistons and seals and bleed valves and a complete system fluid change.
Could it be something wrong with the pad pusher thingie?

[Stick Insect]

OK Dan,

first thing is to check the pads can be pushed in and out of the caliper easily. if it sticks then it will reduce the braking power/efficiency.

Just because the logo/bleed is the right way round does not mean the direction of rotation is correct.

The Audi has trailing calipers ( behind the hub) and I feel the Porsche is the other way round, Calipers ahead of hub, this means that the caliper has the disk rotating through it the wrong way.

I may be wrong...

Look inside the caliper near the name Brembo cast into it and there is a small arrow pointing out direction, this needs to be the same as the normal direction of travel.

I believe the small piston moves sooner and then pushes the front of the pad into position and then the bigger piston moves the pad later. This should stop the squeak.

If you don't cure the squeak you need to learn the following:

Where to Guv'nor?

Do you know who I had in the back of my cab last week?

You could make a fortune round London.


David

[Stick Insect]

Dan
I've just asked and the 968 is a trailing caliper. It would be a good idea to check all the same that the rotation arrow is correct. if it is the pads may well be sticking.

D.

[Alistair C]

David,

From what I've read, the disproportionate piston size is a bit of an old design feature from the days when pads released lots of gas when being used hard. Modern compounds release far less gas than 10 years ago and therefore the need for different sized pistons has reduced probably to a point where thay are no longer needed.

According to the books I read back when I was doing my kit, The bigger piston is always used on the exit of the caliper because the pads generated gasses as soon as they were heated. As the pads are so wide, by the time the second piston was acting on the pad, there was already a gas build up caused by the leading piston. A bigger second piston was then used to overcome this build up & give equal clamping forces top & bottom.

Can't help with the squeaking though! what pads are you using Dan?

[dan_b]

I'm using EBC Reds.

What can be done to prevent/cure pad sticking?

I must admit I wondered why the pistons were different sizes, it seemed odd to me.

[Stick Insect]

Dan, if the pad cannot be pulled in an out easily then you need to sort the pad guides.

The caliper is aluminium and this reacts with the stainless pad guide plate. the stainless pad plates need to be removed 2 x allen screws, scrape and remove the white electrolytic corrosion off, seal ( paint) the affected area, use some sort of anti corrosion coating 9 High temperature silicone grease, copper grease etc.

the best solution is to remove the caliper and bead blast the whole caliper, repaint with 2 pack paint, new seals at about £200 and all will be well, until next time.

The screws have a habit of sticking and so I would suggest using oxy-acetylene to warm the screws before removal.

David

[urq20v]

I had exactly the same squeaking problem with my 993/A8 brake setup.At the suggestion of a local brake shop,I lubricated the ends of the pad backing plates and the stainless pad carriers with Caltex Kopr-Kote grease.Worked like a charm.Squeaking totally gone!

Rex