Brake Pad Diagnostics (long)

DByers · 04-11-2005, 08:31 PM

In getting ready for my track event on the 22nd & 23rd with POC, I decided to change my brake pads out. I have a track day (DE) this Wednesday and I thought the 150 mile drive there, the 5 30 min. or so sessions on the track and the drive home should really embed the pads for my STS event with POC. I plan on taking it easy for the first few track session to make sure the brakes are there.
My brakes have always felt good to me. Firm, easy to modulate and very predictable. The pads seemed to have plenty of life left on first inspection before removal, but I decided to change them any way.
So in this picture is the front passenger inside pad. It has uneven wear at the top compared to the bottom. The rest of the front pads had even wear and as much life left as the bottom part of this pad. The rears also had even wear but had another 3mm or so of life left.
Picture of pad in question

This pad was hard to remove. The others were snug at best. So my question is, could the caliper be sticking on the bottom? Am I going to need to rebuild soon? I put in the new pads with some paper behind it and pressed the pedal several times, removed the pad and paper just to see the mark. It seemed even through out the circle pattern. As stated before, my brakes have always felt great. Ofcourse I have only driven my 911 so I can only compare them to my wifes burb and the company car (Impala).
Opinions welcome.

Oh and I labeled all the pads by location and put then away in a box with the date on it. I figure I can pull them out when I remove the pads for wear in the near future.

jevvy · 04-12-2005, 12:00 AM

Ello mate, I cant really comment on the uneven pad wear however I have had recent experience of sicky calipers that I thought I would share.

I was often having groaning brakes just after I had used them and thought it was time for a caliper rebuild - I spoke to my local P-car mechanic and he said that they hardly ever have to rebuild calipers, they just free them off so I thought I would give it a go.

This is how he told me to do it....
Jack up and take a wheel off
Push the pins out that hold the pads in
Take ONE pad out and insert a pry bar or long screwdriver in its place
Get a friend to press the brake pedal down until the piston moves out of its normal place
liberally spray with WD or other lubricant(I also sprayed a bit under the rubber boot)
Then get yr friend to press and release the pedal as you sit and force the piston back in when he releases the pedal
Once the piston moves freely then put that pad back in and do the piston on the other side of the disc

I did this all around my car and the brakes are now spot on with no draging or sticking.

Good luck mate and sorry if this is teaching you to suck eggs but it really worked for me and I have managed to dodge a rebuild for the time being

Wil Ferch · 04-12-2005, 04:19 AM

Another hint....notice that the caliper puck is not linear across its face....and has a 20 degree angle orientation that must be maintained. The orientation causes a harder push on one side of the pad vs the other...to "even out" the wear of the leading vs trailing edge of the pad. There is a "tool" available that helps you get to the 20 degree angle. You can also make your own or buy one from Mercedes dealers for $10...they also use ATE brakes.

Otherwise....exactly what you show ( wedge-shaped wear) will occur.....

Wil

spydr32000 · 04-12-2005, 04:30 AM

Will, spot on, this is a fairly common problem. The pistons seem to be able to rotate in certain conditions independently. You will also need to check your pistons move freely in the caliper.
Regards Mike

DByers · 04-12-2005, 05:45 AM

Thanks guys. The pistons moved freely to my untrained eye and feel. I will run these pads in for a few hundred miles and pull them and recheck everything.
Wil, I will search for the tool so I have it in my toll box. I didnt see mention of it in my repair books. I guess it just one of those things you learn about.

Wil Ferch · 06:19 AM

Look here for more....Bill Verburg's response --> 930 brake piston orientation

Compare that to the location of the "teeth" marks on the backside of your shown pads in the pics...I think this is the problem....

Wil