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edlithgow
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PostSubject: Dragging front brake   Mon Feb 29, 2016 8:51 am

My drivers side front wheel is getting hot, and the brakes (Daihatsu Skywing FWD with floating single-pot calipers) seem to be dragging. Jacked up with the wheels off at idle in 2nd gear, that side doesn't rotate. Feeding a bit (not a lot) of brake pressure in to transfer some drive to the stuck side gets it moving, so its not VERY stuck, but its stuck enough to be a problem.

Have already cleaned up the slider pins and pad guides, and swapped the pads side-to-side, to no avail. Its got a new brake hose that side, but I've used PTFE tape on the outer threads of both the brake hose and the bleed nipple. I was pretty careful to avoid it getting into the plumbing and acting as a non-return valve, and opening the bleed nipple doesn't free off the wheel, so I don't THINK that's the problem.

So I reckon I've got to take the caliper apart and clean it up (or replace it, IF available). I've taken it off, but have so far not been able to remove the dust boot, though I may have already damaged it trying.

The Daihatsu G100 manual (Not the same car, Skywing seems to be mostly like a G11, but probably close) says:-

"Detach the cylinder boot set ring and cylinder boot, using a common screwdriver"

which seems to be easier said than done.

Other (non-make specific) sources vary, from the realistic/defeatist "cut away the dust seals" (perhaps implying that it isn't possible to get them out intact, at least on Rovers)

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to the standard-issue insane optimism of "Remove the outer dust seal, too." (Yeh, thanks for that piece of "useful" advice)

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Eric the Car Guy seems to leave them in place, and just blows the pistons out through them with compressed air.

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I don't have compressed air, so my options for removing the piston (whether or not I get the dust boot off first) seem to be:-

(a) Take it to a filling station that has air (rather a lot of them in these parts don't)

(b) Find my old brake hose and hook it up (duct-tape may feature) to a bicycle pump.

(c) Put it back on the car and blow it out with the brake pedal.

So, two questions:-

1. Any tips for the disassembly process, especially dust boot removal?
2. Any opinions on improving brake retraction, to avoid dragging brakes?

Eric and several other sources state that piston seal distortion is responsible for pad retraction, though this seems to be controversial.

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My pads have return springs on them, though they don't seem to be very strong.

Would replacing or supplementing these be worthwhile?

I think I've seen some mention of this being done in the context of hyper-mile-ing. Can't find it now but havn't looked very hard since I don't think there were any details in the post.
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Justwatching
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PostSubject: Re: Dragging front brake   Mon Mar 07, 2016 10:59 am

Since you've ruled out the alternative causes, you're looking at either a rebuild kit or total replacement. Either way there should be a new dust seal, so preserving the old one shouldn't be necessary.

If, for whatever reason, the old dust seal must be reused (and compressed air wasn't easily accessible), I'd try the using brake pressure to get it out. A slow 'n' steady approach, fluid catch pan and eye protection recommended.

As for question two, my only recommendation would be regular maintenance. I disassemble, clean and re-lube my brakes every year. I'd put new return springs in if replacements are available, but wouldn't worry too much if not. The failure of the springs should only result in very light drag (assuming the rest of the brake functions correctly).

Btw, do any of the other wheels have drag?
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edlithgow
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PostSubject: Re: Dragging front brake   Tue Mar 08, 2016 8:13 am

Justwatching wrote:

Btw, do any of the other wheels have drag?

Not noticable drag, but I THINK wheels normally (or at least often) have some drag by (poor) design, hence the "stronger return springs" hypermile question.

My understanding is that many disk brake designs don't have any return springs at all, so they are relying on some seal distortion and disk run-out to return the pads. This implies some disk contact.

Drums (apart from very old ones) rely on auto-adjusters which I THINK also rely on some contact (or at least they would if they worked).

Piston is out. Might try a G-clamp to re-assemble it.

Compressed air just sprayed rusty brake fluid around, so I cleaned the caliper up a bit with coke-can in a drill chuck (obscurely satisfying, if pointless, but I thought the vibration might help a bit) then put it back on the car to blow out with hydraulic pressure. When it was about half way, I eased the boot back over the piston, pushing it back with chopsticks (bit of local colour there).

After the first rather sudden "give" I tried to use...er..short strokes, to minimise the risk of master cylinder seal damage.

A lot of gritty, rusty sludge in the caliper cylinder, but it cleaned up quite well. There was quite a lot of barrel corrosion ahead of the seal, and some light pitting of the corresponding part of the piston, but not much corrosion behind the seal, suggesting the sludge came from elsewhere, presumably master cylinder and/or pipework.

The piston had some black "varnish" stuff adhering, which I'd guess was transferred seal material. I removed most of this, and I suppose it MIGHT have been contributing to the sealing, so maybe it'll leak when I put it back together.

I put silicon grease behind the dust boot, but found it impossible to get the piston back in with the boot in place.

I thought of cutting it and using superglue after assembly, (which I'd already used to patch a hole made when initially trying to remove it), but the silicon grease (which is effectively impossible to remove) would probably have stopped that working.

Having nothing to loose, I tried harder, and was eventually able to loosen the circlip from its bed of heavy corrosion and get the boot off, without causing it much more damage.

Scraped out the boot-groove, but have to do some work now so it'll probably be a while before I try putting it back together.

Pictures tell the story. (WARNING: Not suitable for those with mechanical sympathy, like, say, Germans)

Caliper on first removal.

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Back on car during piston removal with hydraulic pressure, after partial clean-up and failed attempts with compressed air.

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Piston on first removal. Euew!

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Bottom of piston on first removal. Euew! again.

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Sludgy Cylinder Base

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Cylinder Rim Corrosion

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Cleaner Piston

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Dust Boot Out

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Dust Boot Seat Corrosion

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Partly Cleaned Dust Boot Groove (Bore looks rusty but its mostly rust from the seating groove sticking to silicone grease)

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edlithgow
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PostSubject: Re: Dragging front brake   Tue Mar 08, 2016 12:13 pm

Occurs to me there MIGHT be DIY possibilities for a supplementary dust seal that would also back-up the return spring function, if you stretched a sheet of elastomer across the top of the piston and anchored its edge around the piston-port perimeter.

You might need to drill and tap the caliper body for an anchor ring though, and you'd need the right elastomer.

Brake dust seals are specced for 120 degrees, which is the top-of-the-range for inner-tube butyl rubber, which therefore probably wouldn't do.
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Justwatching
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PostSubject: Re: Dragging front brake   Tue Mar 15, 2016 11:20 pm

That's an amusing design idea, mainly because it translates in my mind to putting rubber bands around the brakes. I'm not convinced of the design's durability, though I can't see an obvious reason why it wouldn't work in terms of the mechanical principle. Finding the right material would indeed need some consideration. Something stretchy enough to not break when the piston extends, yet go too stretchy and it won't aid the return of the piston.

Or you could buy new return springs!
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edlithgow
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PostSubject: Re: Dragging front brake   Thu Mar 24, 2016 12:26 pm

Put it back together without any extra bits (though I was thinking silicone rubber oven gloves.) Seems much better, and no apparent leaks, but there's a grinding noise from the other (right) side, mostly on left turns.

I had the pads off that side with the intention of stripping that caliper too, but ran out of time and just put it back together again.

Maybe I swapped the pads again and they're just re-bedding, but I wouldn't expect that to be turn sensitive. Maybe the wheel isn't seated home right.

Might have time to look at it some more this weekend.
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Justwatching
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PostSubject: Re: Dragging front brake   Thu Mar 24, 2016 4:57 pm

Good news (grinding noise excepted). I'll admit I was skeptical. I've never had much luck with caliper rebuilds. By the time they became symptomatic, they were too far gone with pitting on the cylinder wall/bore.

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edlithgow
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PostSubject: Re: Dragging front brake   Fri Mar 25, 2016 1:17 am

The general opinion seems to be that this is a waste of time if a seal kit isn't available, so I wasn't optimistic, especially seeing how sludgy it looked inside.

The latter bodes ill for the master cylinder/pipework, which I suppose should be the next job, but I won't attempt a master cylinder rebuild without a seal kit unless I absolutely have to.

In the Yook I'd be looking to replace the pipes with copper (I have/had an unused pipe flareing tool there) but I dunno if that'll be possible here.

The surprising lack of local corrosion may be down to the grease used. Its difficult/impossible to find rubber grease here (as specified by Daihatsu) and I strongly suspect local mechanics use standard lithium petroleum base grease (not unknown with British mechanics, in my limited experience). This'll won't be rubber-compatible, but it'll perhaps  have better corrosion resistance. Maybe with modernish synthetic rubbers the rubber-compatibility doesn't matter.

The general recommendation seems to be to use silicone grease, which is what I used, (bought in Japan), but apparently that has lousy corrosion resistance. When I do the other side I'll use rubber grease, sort of an accidental experiment.
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