UPDATE - Seized caliper piston

dinyull

I have 2 bikes - winter/commuter running disc brakes and summer running rim brakes. The past week I've had to commute on the summer bike whilst I was waiting for new tyre for the commuter. Because the summer bike has been in hibernation I've not really noticed, but back on the commuter this morning my rear disc brake is as good as useless.

I'm assuming that I've glazed the pads, although when originally fitting the brakes I made the same effort with the rear as the front to bed them in and all seemed well.

Am I right in thinking that I can sand the surface of the pads down slightly to take off the glazing, or should I just pull the trigger on a new pair?

Should add, the brakes are Acor (Juin Tech) semi-hydraulic.

meanredspider

Start by trying to de-glaze the surface (you'll need to bed them in again)

dinyull

And how do I go about de-glazing?

meanredspider

Dinyull wrote:

And how do I go about de-glazing?

Sanding them as you suggested.

ugo.santalucia

I'm pretty sure you haven't glazed the pads.
Folks on here seem to think glazing a pad is as simple as flipping a burger

They are more likely contaminated (oil, dirt...)

ANyway, the cure is the same... sand them

dinyull

That was in the back of my mind too. I'm careful (when cleaning the drive chain) to cover up the rear disc, must get some specific disc cleaner though.

cooldad

Dinyull wrote:

...must get some specific disc cleaner though.

Don't. Most are for cars, and contain contain additives which can contaminate bike brakes. Cars are a lot less delicate.
Just use water to rinse them, and Isopropyl alcohol to clean the rotors if they need it.

veronese68

Try just using the back brake hard a few times as if you were bedding them in again. I had to do that with my back brake every now and then. I think I only ever used to use the back brake very lightly which caused the problem, now I tend to use it a bit harder more often just to keep it in good order. Never bothered sanding it.

dinyull

On sat morning I started to look into sanding down the pads to fix the problem. This made no difference, so tried a few other things (replaced cable, played forever and a day trying to centre the caliper) but nothing seemed to work.

Eventually took the caliper off and had started investigating - seem like one of the pistons is seized as only 1 pad is moving. No wonder it wasn't working. I took off the front too just to compare and the pistons are both moving freely on that one.

Didn't really know what to do, so tried soaking it in hot water hoping some gunk was just stuck somewhere, but this hasn't worked.

Any other ideas to free it before I contact the shop to get a replacement?

Ben6899

Dinyull wrote:

On sat morning I started to look into sanding down the pads to fix the problem. This made no difference, so tried a few other things (replaced cable, played forever and a day trying to centre the caliper) but nothing seemed to work.

Eventually took the caliper off and had started investigating - seem like one of the pistons is seized as only 1 pad is moving. No wonder it wasn't working. I took off the front too just to compare and the pistons are both moving freely on that one.

Didn't really know what to do, so tried soaking it in hot water hoping some gunk was just stuck somewhere, but this hasn't worked.

Any other ideas to free it before I contact the shop to get a replacement?

Try soaking it in something other than just water. A degreasing agent like GT85 or Muc Off (if we're going for brand names) should help.

dinyull

Is something like that likely to void a warranty? The caliper is only a few months old, so wary of doing anything that going to cause me problems getting a replacement, if needed.

timothyw

If it's only a few months old, I'd suggest going straight to the warranty replacement route - no point mucking about. It shouldn't have broken already.

Man Of Lard

TimothyW wrote:

If it's only a few months old, I'd suggest going straight to the warranty replacement route - no point mucking about. It shouldn't have broken already.

This

dinyull

I thought that, I'll get onto them.

ugo.santalucia

It's not the first time I ehar of seized pistons on these brakes... I am beginning to have second thoughts on them... maybe I should have gone for the HyRD I originally intended to fit on my SS...

Oh well, fingers crossed it won't be me next

dinyull

Yeah, after a little bit of reading this morning doesn't sound like I'm the first.

Need to speak with the shop, but considering getting a refund and return to the Spyre's I still have in the garage (although the Acor's are better stoppers and less lever travel).

dinyull

And...the other caliper seized up on the way home last night.

tsarouxaz

Dinyull wrote:

I have 2 bikes - winter/commuter running disc brakes and summer running rim brakes. The past week I've had to commute on the summer bike whilst I was waiting for new tyre for the commuter. Because the summer bike has been in hibernation I've not really noticed, but back on the commuter this morning my rear disc brake is as good as useless.

I'm assuming that I've glazed the pads, although when originally fitting the brakes I made the same effort with the rear as the front to bed them in and all seemed well.

Am I right in thinking that I can sand the surface of the pads down slightly to take off the glazing, or should I just pull the trigger on a new pair?

Should add, the brakes are Acor (Juin Tech) semi-hydraulic.

I have heard numerous stories about these brake calipers, most of them horror ones, as I was about to buy a set of them. throw them away and get yourself a set of HY/RDs. they are, in their present version, flawless.

ugo.santalucia

tsarouxaz wrote:

I have heard numerous stories about these brake calipers, most of them horror ones, as I was about to buy a set of them. throw them away and get yourself a set of HY/RDs. they are, in their present version, flawless.

Annoyingly, when I bought them last autumn the internet wisdom was they are a godsend... fingers crossed they won't give me grief... the good thing is that I have them on my single speed, which hardly does any descending

The Rookie

Beware of attributing only one piston moving as the other seized.

If there is no load on the pistons (they arn't squeezing anything) then its vanishingly unlikely the two have the exact same stiction, so one will start to move first and continue to move while the other doesn't, that does NOT mean its faulty.

The way to diagnose a seized piston is if the disc buckles over rather than the second piston starting to move once the first is starting to push on the disc. Seized pistons more often seize out than in as the seal force to retract them is much less than the hydraulic force pushing them out.

dinyull

Cheers for the input, although I'm struggling to understand you sorry.

When fitted and I pull the lever only 1 piston moves. It doesn't move when the other has made contact with the disc - it just doesn't move. There is only 1 pad "slowing" me down.

Same when it's off the bike - only 1 piston will move.

The caliper that is/was working moves both pistons at the same time (on and off the bike).

I literally spent hours on sat trying to fettle and centre the caliper - replacing cables etc.

apreading

The Rookie wrote:

Beware of attributing only one piston moving as the other seized.

If there is no load on the pistons (they arn't squeezing anything) then its vanishingly unlikely the two have the exact same stiction, so one will start to move first and continue to move while the other doesn't, that does NOT mean its faulty.

The way to diagnose a seized piston is if the disc buckles over rather than the second piston starting to move once the first is starting to push on the disc. Seized pistons more often seize out than in as the seal force to retract them is much less than the hydraulic force pushing them out.

^ wot he said. The hydraulic fluid will take the path of least resistance. Its unlikely that both pistons have exactly the same resistance, so without anything to push up against, one piston will almost certainly move before and/or more than the other. If the difference in resistance is significant enough, one may not move at all until the other piston comes up against a hard object.

And as he said, they normally stick on rather than off. It would have to be seriously stuck to not come out.

If that is the case then taking the piston out, cleaning out any debris and making sure it is oiled around the sides and seals (with the same hydraulic fluid) and fits properly so that it moves freely. This is really the only way to be sure it should work fine. When I had some Avid Juicy 3s that constantly gave me problems, we had to hit one piston with a block of wood and hammer to get it to move at all at one point!

The Rookie

If in doubt use a cable tie to hold the moving piston in and then use the lever to force the other out far enough to start cleaning round the piston, I use alcohol at first and then whatever fluid is inside the brake,

skeetam

The Rookie wrote:

If in doubt use a cable tie to hold the moving piston in and then use the lever to force the other out far enough to start cleaning round the piston, I use alcohol at first and then whatever fluid is inside the brake,

I had a really stubborn stuck down piston. I submerged the calliper in a hot water and degreaser mix for an hour, then cable-tied (x3) the piston that moves and after a few lever pulls it started moving again. Once I could see the edges of the piston, I got some WD40 in there and gave it a good clean with a few cotton buds. I washed all the WD40 off with soapy water and refitted. Thanks for the tips in this thread!