[vonny232]

Hi,

I noticed my front left brake disk was hot like a radiator yesterday. Touching the disk with a wet finger would give a "hiss" so the disk was over 100 degrees (the right disk was significantly cooler) so there is definitely an issue.

I took off the wheel and used a crowbar to prise the caliper apart, and then depressed the brake to close it again.

I repeated this a few times until I could slide the caliper open/closed by hand (there was still a bit of resistance, but it felt it was the guide (red circles) or the clips (yellow circles) causing the resistance, rather than the piston itself).

The caliper itself is quite rusty with it flaking quite a lot.

The photo is taken from the internet, as I didnt photo my one as I thought I had fixed the issue.

Question - If the caliper moves by hand (with a bit of resistance, which seems to relate to the guides or clip) and the disk still gets hot, what do people suggest I do next?

[Ennoch]

I've not worked on a sliding BMW caliper before but they're all much the same; if the piston is going back in easily when pried but the caliper isn't moving freely on the pins then it's the guide pins that are the problem. On my old Honda the brakes generally needed servicing in this way every 18 months, at most, as it was that car's one Achilles heel. I'd order up a new hardware kit and some high temperature brake grease. It shouldn't take more than twenty minutes per side to pop the caliper off, pull out the pins, and clean it all up. You may be able to save the pins by cleaning the grease and corrosion off them and then sticking them in a drill with some 320 grit paper to get them smooth again, but it's a bit ghetto and won't last as long as a new set of pins as the scuff marks will allow corrosion and gunk to stick it more quickly. But as a stop gap it's a useful option. Also clean out the holes in the bracket with a fine bottle brush and lots of brake cleaner. Keep spraying it in until it comes out clean. If you take the brackets off then a small circular steel brush cut off so it fits in a drill is very helpful to get the last stubborn crap out but you can have a similar effect with rolled up abrasive paper, it just takes longer. Finally blast it all out once more with brake cleaner before re-assembling with new pins, dust seals and grease. Some cars are a pain to seat the dust seals correctly so make sure they're properly on otherwise all you work will be for nothing after just a few months.

[Billfitz]

That, and you may have to sand rust off the pad carriers as well.

[vonny232]

Thanks for the replies. Why would I need to sand rust off the pad carriers? I thought I just needed to address the areas of linear movement (the rattle clip, guides and pins)?

[Ennoch]

Originally Posted by vonny232

Thanks for the replies. Why would I need to sand rust off the pad carriers? I thought I just needed to address the areas of linear movement (the rattle clip, guides and pins)?

I think he was meaning where the pads fit into the carriers. Some cars have stainless wear plates on those surfaces to help movement, and over time the rust can build up which, as it takes up more space than the steel it's been converted from, can jam the pads. Some abrasive paper should see it good though, followed by a smear of brake grease on the surfaces (or copper slip).

[Billfitz]

Originally Posted by vonny232

I thought I just needed to address the areas of linear movement (the rattle clip, guides and pins)?

The pads move in the carriers. When they can't bad things happen, like overheating the rotors.

[vonny232]

Sorry, I guess its just the terminology im not familiar with.

So in this context, the "carrier" is the smaller of the two cast iron parts that make up the main caliper body?

[Ennoch]

Yes, that's it vonny232. The calipers you have are single piston sliding units which are essentially in two parts; the caliper itself and then the carrier/bracket (I've seen them referred to either way). The caliper then uses the guide pins which are attached to the bracket to slide back and forth and essentially retract both pads from the disc. All of the parts in the system need to be running smoothly to prevent brake bind. On sliding calipers it's most common that either the bit the pads sit in is corroded and binding the pads, or the pins themselves have either become gunked up or corroded. The pistons in the caliper will eventually corrode as with any brake but that's rarer than the other causes.

[Billfitz]

Originally Posted by vonny232

So in this context, the "carrier" is the smaller of the two cast iron parts that make up the main caliper body?

https://www.realoem.com/bmw/enUS/sho...diagId=34_1968
Part #1 is the caliper.
Part #2 is the carrier.

[vonny232]

Thanks folks.

I just watched this video, it suggests that once the rattle clips, then guide pins are unbolted the caliper comes away (giving access to the guide pins themselves plus the rails on the caliper/carriers)?

[Billfitz]

It's hardly unique, every car I've had for the last 40 years was the same. If you're going to do any major cleanup of the carrier you have to pull it. It's too difficult to do a good job of it while on the car.

[vonny232]

It would be the first caliper ive dismantled, I usually get the mechanic to do such things but ive spent far too much money with him lately and this problem looks eminently fixable.

[Billfitz]

On a one to ten degree of difficulty scale brake work is perhaps a three.

[vonny232]

Hi Folks, I got around to taking off the caliper to investigate the sticky front brake caliper.

Despite cleaning the guide pins and runners and applying some grease, the disk is still getting hot (not insanely hot, but hot enough to sizzle a wet finger).

It seems to me the caliper piston has trouble retracting all the way. It seems to be stiff if I try and retract it more than 90% (though it will fully retract if I used a crowbar).

I photographed the caliper (ignore the hammer in the 2nd picture), it seems quite rusty and I wonder whether some rust has crept inside the piston too which is preventing it retracting fully?




As you can see, I sanded and greased the guides, which hugely improved the sliding performance, but only once the piston was retracted.




Question - Is it likely the piston is goosed, and if so, is it worth trying to repair or should I bite the bullet and source a used one and have a mechanic replace it?

[Ennoch]

It's very possibles as the caliper does look rusty, especially around the boot seal. You therefore have a few options (depending on your ability);
- buy a seal kit and piston and rebuild yourself after pinching off the brake line to minimise fluid loss (have a mate on hand to help re-bleed the caliper). Top tip; try cracking the bleed nipple off before doing anything as there's always a risk they can snap. Soak a bit of oil around the nipple overnight before trying though.
- Buy a new/refurbed caliper (usually you can use your existing caliper as an exchange) and fit it yourself.
- Take it to a garage and get them to put a refurbed caliper on there.

Personally, if you're having to pay for labour then I wouldn't bother with a used caliper. It won't be much less than a decent refurbed one and that comes with a guarantee rather than a total unknown with the risk your 'new' used caliper won't do as yours has done in a few months time.

[vonny232]

Hi folks, in case anyone else had my problem...

The rubber dust boot was wrecked, so I ripped it off and there was rust on the piston and surrounding flange, the piston was also bone dry (no grease).This would explain the sticky caliper.

I cleaned up the surfaces (bearing in mind the surface of the piston cannot be scratched), polished it the best I could and then refitted a new dust boot. This seems to have fixed the problem.

Cheers.