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Author: Subject: Sierra rear brake caliper - piston wind in/out
Crispy

posted on 6/5/16 at 08:27 AM Reply With Quote
Sierra rear brake caliper - piston wind in/out

Hello All,

In brief, my car (Raw Striker) failed its MOT last week on two points. The first was that the hand brake cable failed during the test (now sorted and properly adjusted). The second was that when testing the foot brake there was an imbalance in braking force across the rear axle. The tester also pointed out that there was fairly heavy corrosion on the inside face of the rear disc on the drivers side (the side of the disc facing the piston of the caliper).

I set about checking the drivers side brake caliper, found that the piston was very reluctant to move but have now tackled this and it moves freely.

The rear caliper on the passenger side, the side that I had assumed was working properly (it has the higher brake force when applying the foot brake, locks more quickly when applying the hand brake and has even shiny surfaces on both sides of the disc) has a strange problem. I was checking how well its piston was moving and found that although I can easily wind this piston back into its bore using a wind back tool, it point blank refuses to wind back out again. It's not that it doesn't extend when wound, its just that the piston refuses to rotate anti-clockwise (although it will easily slide out of its bore if I apply the foot brake). I've tried applying higher torque to get it to turn anti-clockwise up until the point where I thought that going any further was likely to damage something. It's strange as winds clockwise very easily.

I take it that this isn't normal but was hopeful that you might be able to offer some advice.

I also found it strange that the corrosion on the drivers side disk was bad on the piston side. I was expecting that on a floating caliper brake like this that a problem with the sliders would lead to poor pad contact on the outside face of the disc and a problem with the piston would give poor (but even) pad contact on both sides of the disc. The pads themselves were free on both sides of the disc with no signs of binding at any point along their travel.

Any input will be gratefully appreciated.

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Slimy38

posted on 6/5/16 at 09:17 AM Reply With Quote
I believe that's actually correct? Certainly all the calipers I've worked on that need to be wound back will only wind one way, even brand new ones. I've done the same as you, I've applied quite high torque and it's not budged one bit.

To move them the other way just apply the brakes, and as long as it slides out reasonably easily then I'd be ok.

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Crispy

posted on 6/5/16 at 09:53 AM Reply With Quote
Only just realised after reading through my first post that I wasn't exactly clear that my two rear calipers now show different behaviour - the piston in the drivers side caliper will wind in with a clockwise rotation and out with an anti-clockwise rotation, the passenger side caliper piston will wind in with a clockwise rotation but will not wind anti-clockwise at all.

Looking at chapter 10 of the Sierra Haynes manual - Braking system, Section 7 'Rear disc caliper - removal overhaul and refitting' after removing the hand brake cable and the hydraulic brake line from the caliper, point 7 (disassembly) reads:

'Rotate the piston anti-clockwise, using Ford tool No 12-006, or a pair of circlip pliers or similar tool, until it protrudes from the caliper bore by approximately 20.0 mm (0.8 in)'.

This suggests that the piston should wind in with a clockwise rotation and wind out with an anti-clockwise rotation. As I've now got one caliper that exhibits this behaviour and one that doesn't I have to assume that one of them has a fault. Unless it's possible to identify which one is showing the incorrect behaviour I guess I'm going to have to get two refurbished calipers and replace them both.

Should I just disregard the info in the Haynes manual? One of the owners clubs that I'm a member of refers to the manual for that car as the 'Haynes book of lies'.

[Edited on 6/5/16 by Crispy]

[Edited on 6/5/16 by Crispy]

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Grimsdale

posted on 6/5/16 at 11:09 AM Reply With Quote
if you look on the back of the pad, there is a lump sticking out which ties in with a cutout on the piston. Therefore in use, it is impossible for the piston to rotate as it comes out.

I suspect your piston that rotates as it comes out may have a faulty handbrake auto adjuster, and the part that allows it to slide out without turning has seized. Replace the caliper on the side that was identified as faulty on the MOT

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Crispy

posted on 6/5/16 at 11:50 AM Reply With Quote
Thanks to both Slimy 38 and Grimsdale.

I'm inclined to agree that it's the caliper with the piston that will wind both in and out that is at fault as it's the one showing the uneven wear on the disc. The only reason for not just going with my own judgement and replacing that one was what was written in the Haynes manual. I should learn not to believe everything that I read.

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hizzi

posted on 6/5/16 at 12:15 PM Reply With Quote
sorry but its brakes on a sports car, change the calipers as a pair. there not exactly big cash these days
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Matt21

posted on 6/5/16 at 03:09 PM Reply With Quote
I can confirm that the rear pistons should NOT wind out.

I have recently had mine off to clean them up etc and they only wind in.
I'm not sure why one winds back out, but it shouldn't.

No need to change both though. Pads and discs I would say change both, but calipers?
If one is working fine why change it for another that it also working fine? It's not like they're made as a matching pair.





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Crispy

posted on 12/5/16 at 11:14 AM Reply With Quote
In the end I ordered two TRW refurbished calipers for the car.

Interestingly the pistons on both of the refurb units wind freely both clockwise (retracting into caliper body) and anticlockwise (extending out of caliper body).

The piston of the old caliper from the passenger side of my Striker also freely winds in both directions now that itís off the car. Maybe there was still a bit of tension in the hand brake cable when I tried this before or maybe Iíd set the wind back tool too tight in the caliper before trying to extend the piston with and anti-clockwise rotation (doubtful as I performed a clockwise rotation prior to trying an anticlockwise rotation and I donít believe I tightened the tool in the caliper in between)- the pitch on the thread inside the caliper is more coarse than the thread on my wind back tool, when you wind the piston clockwise the piston retracts more quickly than the head of the wind back tool causing the tool to become loose in the caliper body, when turning anticlockwise the piston moves out more rapidly than the head of the wind back tool retracts, causing the tool to become wedged in the caliper body and the tool itself can then prevent rotation in this direction.

So thatís 4 calipers in my possession at the moment with all 4 of them allowing the pistons to be freely wound in both directions when not fitted to the car.

[Edited on 12/5/16 by Crispy]

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