Rear Brake Piston - Twist Type

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I just changed out my rear brakes tonight. No biggie, borrowed the tool from Autozone to do the job but as I changed the pads I was trying to figure out how they work. I've changed out the pads on several cars but have not encountered this type of piston before.

So, replace pads, turn the piston clockwise to compress. The piston has four impressions at the 12, 3, 6, and 9 o'clock positions on the face of the piston. The pads have a little raised bump on the lower portion in the middle. As I replaced the first pad, I realized I had to not only compress the piston but get the impression on the piston to line up with the raised bump. The caliper would not go on unless the divot on the piston was at the 6 o'clock position.

Do the pistons turn counter-clockwise as you push the brake? If they do, I don't see how they would turn as this little bump would prevent that. I am assuming there is a sleeve in the piston allowing it to move forward/back like the front piston and the clockwise/counterclockwise turning of the head would just be an adjustment of some sort.

Anyone understand the mechanics of this type of piston?
 
There is a scroll gear on the inside of the caliper and is connected to the piston. The gear will turn just a bit every time the emergency brake is applied which pushes the piston out as the pads wear.
 
Thanks Harry!

So when the brake pedal is pushed...the piston acts just like the front brake piston, moving in and out of the caliper with no turning action, correct?

If the piston does twist, how does the piston get past the nipple/notch on the brake pad?
 
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The piston does not twist during normal operation. You only twist it to "screw" it down against the parking brake mechanism. The notch/nipple is there to specifically keep the piston from twisting during normal operation.
 
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Thanks! I googled around and also found this:

Quote:
No the pistons to not rotate as they uncompress. They simply are pushed out of their bore by hydolic pressure (when you step on the brake pedal) or by mechanical means (e-brake). They only rotate to leave room for new pads. Rotate them till they bottom, then turn them back till they line up wiht the nipple protruding from the pad (YOU DONT CUT THAT OFF!!).

http://www.rx7club.com/showthread.php?t=721890
 
Originally Posted By: 2002 Maxima SE
Anyone understand the mechanics of this type of piston?

I took some pictures when I rebuilt the rears on my Honda.
http://www.tegger.com/hondafaq/misc/rear-caliper-reman/
There are two sets of pics, one large-size and one small-size. If you're in a tight-bandwidth situation, view the small pics.

The piston does not rotate. Instead, there is a worm-screw attached to the piston, and THAT does the rotating. There is a strong spring, held in by a snap-ring, that keeps the screw seated against the rear face of the caliper bore (actually against a movable plate that's inside there). There are bearings that allow the screw to turn even though there is spring-pressure against it.

When you step on the brake pedal, these things happen:
1) hydraulic force acting on the piston pushes the piston forwards, pulling the worm-screw with it;
2) the spring-pressure opposes the piston's direction; the worm-screw rotates against the spring-pressure, backing out of the piston. This is what provides the adjustment for pad-wear. This is also the reason you need to screw the piston in again, to undo all the backing-out that the screw has been doing.

The parking brake acts on a movable plate that is BEHIND the screw. As this plate is shoved forwards by the mechanical parking-brake linkage, it pushes on the back of the screw and pushes everything forwards. The screw cannot rotate back into the piston because it is working against the mechanical disadvantage created by the angle of the screw threads, plus it's sitting against the movable plate.
 
Thanks paul and Tegger!

I get it now...as I push the brakes...the piston moves outward by hydraulic pressure but also repositions the worm screw. As I let off the brakes, the piston does not return all the way inward to the caliper. The nipple on the pad prevents the piston from returning completely inward by holding the head of the piston as the worm screw turns. Much thanks!
 
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It will take a number brake applications to get things normal.
Add this to seating in new pads, and that means take it easy for a few miles - be on guard for odd pedal feel and poor braking.
Pump them before driving - this will help.
 
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