What the Caliper?

[dholling13]

In need of some Dorki help. 4 months ago I purchased from Tischer new Big Blacks (big reds that came on the 928 GTS). Over the last 4 events I have ruined a new set of pagid's and are working on destroying another set.

The pad killer seems to be that the lower most piston or the trailing piston doesn't engage as quickly as the upper or leading piston causing the pad to wear at an accelerated rate from top to bottom.

I have just rebuilt the calipers with new seals and pistons and only one piston showed any wear. Flushed the system with Motul's RBF600 and topped off the reservoir after the ABS pump turns on with SRF...

Drive ability wise, the pedal is rock solid and the brakes stop great. What else can I do?

[Jazzbass]

Hmmm... uneven wear on the pads... sounds like you might have installed them upside down. I'm not talking "bleed screws on bottom" upside down, but rather "big piston first, little piston second" upside down. In the direction of rotation, the rotor should turn past the small piston first and then the large piston. It's a common mistake when adapting brakes from a car that mounts the calipers in a trailing position (behind the hub, like the 928 does IIRC) to a car where the caliper mounts in a leading position (in front of the hub, like most 911s).

If this is the case, you need to re-orient the caliper to work with a leading mount by switching the crossover tubes and bleed screws end-over-end on the caliper, then swap calipers L-R on the car. Typically Brembo calipers have an arrow on them that indicates the direction of rotor rotation through the caliper. If your calipers are in front of the hubs and the arrow is pointing up, they're on wrong.

If this isn't the case, well, you've stumped me outside of saying you have defective calipers.

[OldTee]

Jazz sounds like he knows what he is talking about, but just in case, try a post on the 928 Rennlist board. They are pretty active over there on those cars and somebody might be willing to answer a 911 question.
ARF

[Geoff Daniels]

I believe Jazz got it exactly right. I have Big Blacks on my car but it is a 944 so they mount in the same position as the 928. If you did not swap the bleeder screws with the crossover tubes then you would have had to put the calipers on the wrong side which would result in their being upside down.

Did you really rebuild brand new calipers?

[dholling13]

I believe Jazz got it exactly right. I have Big Blacks on my car but it is a 944 so they mount in the same position as the 928. If you did not swap the bleeder screws with the crossover tubes then you would have had to put the calipers on the wrong side which would result in their being upside down.

Did you really rebuild brand new calipers?

Son of a _______________...

This is exactly the problem; the larger piston is leading.

[Trak Ratt]

Looks like the Jazz man scores!

[Jazzbass]

Son of a _______________...

This is exactly the problem; the larger piston is leading.
Thought so. Some advice - be sure to use a flare wrench when taking off the crossover tubes. A regular wrench will just round over the flare nut - they're usually in there pretty tight. Lots of guys learn the hard way that years of steel threads tightened against aluminum threads don't come apart easily. You calipers are new so it might not be an issue, but better safe than sorry. Don't want to make a $50 mistake and 5 days before VIR.