Faulty Caliper? (How?)

[AnthonyJUK]

Hi,

This one is driving me crazy. My rear right disk gets way hotter than any other. But it's not dragging or sticking.

Under no/light braking, all brakes are about 40-50c.
Medium to heavy three are 70-80c, rear right is 220-230c.
Ultra heavy braking both fronts are 160c, both rears are 250c. (I think this can be excluded as all four will be massively saturated with heat).

I've take the rear right apart. The piston moves freely. The pads move freely*. The sliders are practically factory fresh. Lubed it back up and still the same issue. With the wheel off there is only slight contact with the disk but I've done 5-6 car brakes before and this felt no different from any other. I can easily rotate the disk by hand with the brakes off.

Should I be worried? Or just get new pads & disks? I figure it may have been sticking at some point and it's been so heat damaged that it's now inefficient, and new pads/disks will cure it.

*I didn't notice any difficulty pulling the pads out but the outside pad took some persuasion to go back in. I can still move it in/out by hand though.

Thanks

 

[Retroman]

The discs look old. The rust lip at the top is far too wide and the pads look damaged. I would guess that the surface of those discs is not even hence irregular pad/disc contact. Personally, I would change the lot. I would get non-budget pads which would normally come with some new stainless steel shims against which the pads slide.

 

[Marky007]

Disk could be warped they do look a bit worn . . . .

I would day new disks and pads all will be well . . . .

 

[ruairi83]

Replace discs and pads and you should be good.

If it turns out it’s caliper I have a Brand new genuine one here with motor, carries etc and.

https://www.audi-sport.net/xf/threa...-carrier-q5-8r-a4-8k-b8-allroad-a5-8t.391915/

Ecp is £320 on exchange basis for non genuine caliper so this is bargain.

 

[AnthonyJUK]

These disks/pads aren't that old, only about 20-25k miles. The other side is mint. I always buy branded and have some more on the way. I don't think it's warped as I can't feel anything - though it's harder to feel the back rather than the front. Defo been through too many heat cycles though. It's blue on most of the disk.

It's just that it's odd that one side is generating heat and wearing much faster - whilst the two go hand in hand I'm unsure of the actual initial cause.

And you search the internet and all the results say you need new calipers/rebuild etc etc but mine seem absolutely fine except for the obvious disk/pads.

If there isn't a consensus to troubleshoot further I'll just stick the new pads & disks on. I just didn't want to do it if there is some common failure point that could be causing this.



PS Whilst typing this I do remember a year ago at it's MOT asking for a look whilst it was on ramps as one of the brakes was squeaking, sounded like a stone got stuck, but nothing was found and it stopped a month later. Possible cause I guess if a stone has been stuck for a period of time and caused uneven wear.

 

[B5NUT]

Looks like the caliper is intermittently sticking, as it's the only way you are going to generate temperatures of 250.. Only thing thing that rear disc is now good for is the bin. If it was my car the rear discs and pads would be changed along with that side caliper. I would also re-grease the slider pins as the grease will most likely not rated for that level of temperature.

 

[CHEZ]

How did you notice that one rear disk is warmer than the other in the first place?

 

[AnthonyJUK]

It may be sticking but I can find nothing that is causing it to stick. Not saying it ISN'T, but I've had it in pieces and nothing seems to be a sticking point.

250 deg's is easily achievable by braking downhill from 100mph 10 times in a row! I just did that to see what temps I got but as the original post all 4 were probably totally heat saturated by that point so a pointless comparison. So I wouldn't say the 250c is an indication, I'd maybe agree with the 220-230 medium braking scenario but not the ultra ULTRA hard braking test. Though I think I will regrease the sliders, good idea.

Thinking about it, regardless of the original cause the pad is obvs only partially in contact with the disk now so that would explain the current symptoms.

I have new brakes on the way, unsure of replacing the caliper just yet as so far I haven't found anything untoward and no one has chimed in with a specific cause. Everything is free, no resistance whatsoever. Just one of those things I guess. Faulty batch/Faulty installer (me!)/stone stuck/Devine Intervention.

And I noticed as I got out and thought it was doing a DPF regen as the rear was fooking warm but on closer inspection it was the right rear brakes. The wheel itself (the alloy spokes) were 105c and the sidewall was 70c! (edit to add, it was midnight in North Yorkshire country roads so I was spanking it)(Edit edit, I have a thermal camera/gun type thing at home so I checked out of curiosity)

 

[B5NUT]

When you say you have had it in pieces, are you talking about the caliper, removing the piston etc?

 

[Retroman]

When brake hoses get old, they break down on the inside causing a flow restriction the symptoms of which can be similar to a sticking calliper. Whilst I don't want to bang on about it (and appreciate we are talking at "arms length" on the internet) those discs look worse than most 20,000 mile discs I've seen. However, I appreciate that cars in Scotland and parked close to the coast can suffer a lot of rust/salt corrosion.

You can save money by just buying new seals and dust covers - several "how to" videos on Youtube

https://brakeparts.co.uk/

 

[CHEZ]

It's unusual for a rear, not impossible mind to seize.
It's normally the fronts that seize.
By seize I mean the slides get corroded and hamper the movement.
It could be an e brake thing perhaps?
Have you had an error code scan at all?

 

[CHEZ]

When brake hoses get old, they break down on the inside causing a flow restriction the symptoms of which can be similar to a sticking calliper. Whilst I don't want to bang on about it (and appreciate we are talking at "arms length" on the internet) those discs look worse than most 20,000 mile discs I've seen. However, I appreciate that cars in Scotland and parked close to the coast can suffer a lot of rust/salt corrosion.

You can save money by just buying new seals and dust covers - several "how to" videos on Youtube

https://brakeparts.co.uk/

Good points there regarding oxidisation and paggered disks.
Salt is bad m'kay

 

[CarbonMike]

Christ that disc and pad have seen better days for sure lol

 

[spartacus 68]

Definately replace the discs and pads, no question. A lot of surface corrosion, lip on the disc, etc.

Back to the caliper. Look at a rebuild with kit from Bigg Red. Could be moisture getting to the piston because of damaged 'o' ring, brake fluid change every 2 years helps. Also look at brake sliders and use proper grease. Inspect brake slider rubber sleeves for any sign of damage.

 

[AnthonyJUK]

Thanks for the input.

For sure I was going to replace the discs & pads but I wondered if there was a reason this side has worn faster than the other, or if there is any other reason temps are massively higher than the rest.

Long story short, I think it's simply that they are old & knackered and there is no particular reason that one side runs this hot other than they need replacing, and I can't find anything else to worry about so I'll just stick the new ones on.

To answer a few q's (just for jollies!) checked my receipts and they are 3 years, 30k miles old.

Coming to think of it about 3k was properly hard on the brakes. Racing home over the back roads from a new job... first time I've ever actually had brake fade from the fluid boiling. Funny how you always think "maybe this is brake fade" when your brakes get a bit hot but when you actually get it you know for SURE what it feels like! The same 3k miles also killed the engine (HPFP, that was fun to replace!) so yeah 30k with a chunk of motorway miles and 3k of what I'd say was track day pace... And I haven't driven it much the last 2 years so the discs are just over 3 years old.

There are no codes and the parking brake works fine/doesn't drag. It found it's "bite" point fine when I put the old pads back on and calibrated the e-brake. And after engaging/disengaging the parking brake a few times I could still rotate by hand. So I don't think it's that.

Anyways looking at the new pads I'd say mine were at least 2/3 worn, more like 3/4, without comparison I guessed mine were only 40-50%ish worn. So yeah, way more past their best than I thought.

 

[AnthonyJUK]

Oh also just to add, the reason I was initially taken aback and surprised was because it's just one side that's this bad. But again no point troubleshooting a none-issue, I'll replace the bad pads/discs and monitor.