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Caliper pistons

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Just replaced all the seals in the front brake calipers and was slightly curious as to the reason why the piston faces have only 70% contact with the back of the brake pad? Is the machined out portion for vibration reduction/self alignement or something a little more technical?

Ian.
1987 3.2 Targa.
 
Interesting point, I did the same about 2 years ago and had similar thoughts. I think its to do with the loading on the leading edge of the pad and anti squeal, the alignment of the piston is closely detailed in the seal kit. However, I might well be wide of the mark, wouldn't be the first time [:)].
 
Makes sense Phil, I understand your concept. Think I managed to get the 20degree angle pretty close, don't know what the penalties are for being wide of the mark? Piston & bore wear possibly, maybe vibration too? Anyhow, the brakes are now much improved after the rebuild & gentle bedding in.

Ian.
1987 3.2 Targa.
Audi A6 1.8T.SE.
 
I'd imagine that there's a fair degree of tolerance in it, I went to the fun of making a gauge out of a plastic icecream container however!! Supprising the difference that new seals and fluid make.
The only issue I had was that the new seals kept pulling the pistons back too far, made for a long pedal travel. I ended up taking the pads out, pumping the pistons out then pushing them back until the pad would just go in, problem solved! Had me scratching my head for a while.
 
Does sound strange! Almost synonymous with a vacuum type effect? Have to admit that my pedal travel is marginally longer than before but the efficiency is definitely 100% better and the bleeding was very thorough so we'll see how things "bed in". Think I may give the rear calipers the same treatment next...Strike while the iron's hot!

Ian.
1987 3.2 Targa
Audi A6 1.8T.SE.
 
Arrrh, that's probably why I had more of an issue with it, I did all 4 calipers! Bed in the fronts first and I'd imagine you won't have an issue.
 
Thanks for the info' Phil, will post when I take the next step with the rears. Rumour has it these are more prone to siezing so could be in for yet more improvement.

Ian.
1987 3.2 Targa
Audi A6 1.8T.SE.
 
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