Simon's new Xantia V6 and Leaf blog

[Mandrake]

It was the thin steel backplate (arrowed) that I was thinking ought to be wax protected, both sides.
They have a habit of progressively rusting away, especially in salty conditions, and have been known to be annoying MoT failures.

Ah. What you call backing plate I would call a stone shield.

It's not actually made of steel it's aluminium of some sort! Not easy to see in the photo but the bit of it that the bolts for the caliper assembly pass through that help clamp it into place are clearly aluminium coloured.

I'm not sure how it would be an MOT failure either as it's only a stone shield ?

[white exec]

Simon, it's almost certainly steel, maybe plated/galvanised. Already rusty in your photo.
Although not structural, there have been reports on forums of corrosion of these resulting in test failure.
Easy to prevent, while it's still in one piece.
I appreciate waxing and rustproofing is not a popular activity!

[RichardW]

Removing the discs from the front of our Picasso required an angle grinder, a large hammer and a bolster chisel! They were not for coming off - although I think a cold chisel in the back right next to the hub would be more effective than just smacking the face of the disc.

[white exec]

...which is where a thin smear of copper grease on the hub/disc joint comes in. Doesn't help original fixings, although maybe a drenching of penetrating oil overnight might help, round the periphery and through the stud/bolt holes. Need to be careful with a cold chisel in the 'gap'; if burr not totally removed, disc will not sit flat.

[daviemck2006]

I once had to smack the discs on a focus as hard with the lump hammer I broke chunks out of the discs! I don’t do finesse getting discs off lol

[myglaren]

Changed quite a number of discs and never had to resort to violence to remove the old ones.
The worst were the Xantia ones.
Bought all the kit, took the wheels off and was faced with a torx screw!
Had no torx drivers so went aii the way back to the shop I bought the discs etc. from (almost a mile) and he let me have a £17.50 set of wrenches for a fiver as I was "such a god customer".
Never been there before and he was most apologetic that the discs were £19 each. My daughter had just paid £50 each for Sierra discs!

[CitroJim]

...which is where a thin smear of copper grease on the hub/disc joint comes in.

Always have done that... Works a treat when you need to pop a disc off second time around...

Need to be careful with a cold chisel in the 'gap'; if burr not totally removed, disc will not sit flat.

Good point and it will result in the feeling the disc is warped...

[Mandrake]

I suppose I should thank my lucky stars that the front discs were such an easy job this time.

Honestly, if it hadn't been for the pad ears needing filing it would have been a walk in the park - and that wasn't the fault of the car itself. I don't have a grinder so I had to file them by hand, and I was trying to do so carefully so as not to take too much off and also try to get all 4 of them so they moved with the same degree of freedom, so that filing and checking then filing again process took a while.

I completely coated the face of the hub where the disc presses on with a thin layer of copper grease, as well as on the edge of the hub, so the discs will hopefully come off easily next time as well!

Will there be a next time in my ownership ? Quite possibly! I'm currently doing over 12k miles a year and intend to keep the car at least 4 more years so that's 48k miles, so it may well need another set of discs and pads within that time... not from wearing down to minimum thickness, but from corrosion if the first time is anything to judge by!

[bobins]

Ah. What you call backing plate I would call a stone shield.

I'm not sure how it would be an MOT failure either as it's only a stone shield ?

Having just this minute come from having an MOT at my tame tester, I asked him about disc brake back plates, and to paraphrase, his interpretation is that they only become testable if they are a structural component, or actually hold a brake component, or are proving a danger to other components i.e. going to chafe through a brake pipe.

[white exec]

As I said, anything to actually avoid protecting against rust!
The back plate also has the function of preventing road debris and mud from getting into the disc brake mechanism, which seems to me fairly important.

[Mandrake]

Ok these rear brake will not go willingly...

Some time after the auto-adjusters in the rear drums tightened themselves up, I started noticing the rear brakes (mainly the left one) being very grabby again both when applying the footbrake and when pulling on the handbrake, so I know its definitely the rear brakes.

So much so that I'm having to modulate the brake pedal very delicately when pulling to a stop to avoid an abrupt, over dramatic stop as though I'd slammed the brakes on.

So I'm trying to figure out why it has come back and what is causing it. All of the leading and trailing shoe edges have been carefully chamfered, everything is cleaned and lubricated, the only thing I can think of that might be causing it to grab is that the shoes may still be snagging slightly on a groove on the ears on the backing plate.

People may recall I mentioned the whole backing plate seems to have a hard black coating of enamel paint (?) or something similar, including the raised ears, and some of the ears - mainly the ones near the top (cylinder) side that move the most have a grove worn right through this "paint" to the metal, creating a groove that the edge of shoe may snag on. Here is one example:

I put plenty of copper grease on the ears and it seemed to help a lot but I can only assume the shoes are still snagging slightly on these grooves in the paint, thus allowing the shoes to get stuck so the leading shoe is sometimes doing all the work, thus grabbing. The grabbing isn't always there so it makes me wonder if when the shoes move over a bit to be more centralised it stops grabbing.

So my question is - should I be wire brushing all the paint off the raised ears back to bare metal and then applying a new layer of copper grease so that there are no lumps/groves for the shoes to catch on ? Are the shoes temporarily being biased towards the leading shoe instead of equalising all that it would take to cause the brakes to become grabby and oversensitive at low speeds ?

I can't think what else it could be as everything else is clean and well lubricated and the shoes are in perfect condition, no problems with the cylinder, self adjuster etc...

[white exec]

I'm presuming the shoes rest on these 'ears', biassed in place by the short springs/washers on the front face of the shoe.
I see the ears seem to have a sharpish edge* - has this worn a groove in the back face of the shoe, which could (as you say) make the movement of the shoe snaggy?
Classic arrangement is not an 'ear', but a steel "post" (about 8-10mm or so diameter) which the shoes rest against. This arrangement used on lots of cars.
I snagging is the issue, and the shoes need something less sharp/pointy to rest against, it would be simple enough to fit a bolt (and a couple of nuts) through the back-plate, using the bolt-head to rest the shoe back face on.

Maybe.

* I don't think paint (or not) on the ears can be the issue. Grease there wouldn't last anyway, as it would scrape through.

[bobins]

I'm presuming the shoes rest on these 'ears', biassed in place by the short springs/washers on the front face of the show.

Is it that these springs are, basically, too strong ? They need to hold the shoes relatively firmly in place, but scraping away at the paintwork to cause them to stick isn't normal. I think I'd be tempted to clean up the area with a bit of wet 'n' dry so the shoes can move freely. The other thing - are the pull-off springs routed correctly and the right way up ?

[Mandrake]

Maybe I'm not describing it well. The backing plate has raised sections for the edge of the shoes to rest and slide against. There are three for each shoe.

These are roughly oval shape and originally would have had a painted finish. The sharp edge of the shoe seems to have cut into the paint on these raised edges and has thus worn a groove right down to the metal, and it seems like the shoes may be snagging on this ?

Personally I think the springs (with the retaining discs) that pull the shoes in against the backing plate are far too strong - but they are what they are. If the shoes are snagging on the groove cut in the paint I need to get those raised sections perfectly flat again - either by repainting them or sanding them back to bare metal. (which is probably ok if there is copper grease there ?)

One other thought is perhaps one of the shoe's linings is snagging on a rusty edge of the drum as it rotates around ?

We have heavy snow here this morning and the rear right drum was locking very easily on me and it was getting extremely frustrating as I could not pull to a gentle stop without the rear left wheel locking without warning, going into a slight skid on the snowy ground. I'd very gently easy the brake on as I was coming to a complete stop then "wham" the left wheel would suddenly lock up. We're talking at about <5mph here.

A couple of times it wouldn't let go again when I released the pedal, I had to push the brake pedal hard and then release it then it let go. This is what makes me wonder if the shoes are not free to centre properly due to snagging on the backing plate.

Does anyone have any reasons why I shouldn't sand the raised guides from the backing plate back to the metal ? It's already scraped through to the metal anyway...

Regarding the return spring and auto adjuster spring - I took photos of how they were fitted before I started and they are back in the right way. They can't really fit any other way.

[white exec]

The short (hold-back) springs and slotted washers are there to do one main job: to stop the shoes wandering in and out, and to allow them only to move in one plane radially outwards, against the drum surface. The short springs also act as anti-rattle devices.

The pressure the hold-back springs exert is normally far less than the return (pull-in) springs that pull the shoes clear of the drum when the brake is released.

The hold-back springs will pull the shoes towards the backplate, where (normally) one of two arrangements are designed:
- either one or more raised flat pads, which the shoe edges slide across (when the brakes are applied),
- or a pillar/post/bolthead standing proud of the backplate, which contacts not the shoe edge, but the back side of the shoe.

Either way, the shoe must be able to slide easily across the supporting pad or post, and not jam on it.
A couple of web pics...

Three supporting flat pads (greased)

Pads (3 each side) shown by red blobs

A bolthead/post support is usually located close to the hold-back pin (to avoid pivoting) so that it slides across a flat part of the back of the shoe (not the shoe edge).

The "ear" arrangement you have looks cheap and cost-saving, and definitely not a long-life flat surface. If there is only one of these for each shoe, you could fairly simply add a small flat plate (of the same height) to replace it . . . maybe one small plate each side of the old ear, secured to the backplate by a well-countersunk screw (or rivet, or weld) - or thread the plate and bolt in position from the rear, if the pad were thick enough. Just an idea. A thick washer secured in place might also be possible.