Disc brake suddenly locked up

julianhj

I have a 2010(?) Cube Ltd Race with Formula RX brakes. I used my bike for the first time in a few months this morning. I checked everything over before setting off - all good. I rode a mile or so to my LBS, then on for another mile or so in to town. After a quick errand at the bank (took the bike in with me) I noticed the rear wheel was locked up. I went outside, had a closer look and the brakes were almost fully on. I tried the lever and it was extended at it's normal position; there was very little movement as the pads were already touching the disc.

I got the bike home (took the rear caliper off to get back) and had a Google. I took the pads out, checked that the lever was moving both pistons (it appeared to be), then eased the pistons back with a screwdriver. I didn't use too much pressure, as I didn't want to cause any damage. I put the pads back in and the pistons have only retracted as far as shown in the photo. The disc is a little thicker than that gap, so the pads need to be about twice as far apart I'd guess.



Am I not using enough pressure to push the pistons back? Is there a fault elsewhere? I'm assuming that something has gone wrong somewhere for the pistons to suddenly fail to release.

Can anyone point me in the right direction in terms of a DIY fix, or is this a job for the chaps at my LBS?

zx6man

The pitons should go all the way back I reckon (well, on my motorbike they do). Chances are you need a bit more pressure

CitizenLee

Try a ghetto bleed on it... zip tie the brake lever back to to your bars and leave it over night. I don't know anything about the science behind why it works but I've done it for years and it does make brakes feel better

cooldad

Lever them back properly. Leave the pads in so you don't damage the pistons.

The Rookie

Totally the wrong way round, that advances the pads, nothing like bleeding AT ALL.

Check to see what happens if you push each piston in in turn, if one can go in further while the other comes out, then it sounds like for some reason the system is overfilled, be it with fluid or air, so a following the correct bleed/fill procedure may well sort it.

julianhj

cooldad wrote:

Lever them back properly. Leave the pads in so you don't damage the pistons.

Please excuse my ignorance, but what's the best technique for this?

cooldad

The Rookie wrote:

Totally the wrong way round, that advances the pads, nothing like bleeding AT ALL.

Presume you are referring to the post before mine?
Just to save confusion.

julianhj wrote:

cooldad wrote:

Lever them back properly. Leave the pads in so you don't damage the pistons.

Please excuse my ignorance, but what's the best technique for this?

Stick a big screwdriver between them and twist like a key.

The Northern Monkey

would reccomend using something like a tyre lever, not a screwdriver.

nicklouse

julianhj wrote:

cooldad wrote:

Lever them back properly. Leave the pads in so you don't damage the pistons.

Please excuse my ignorance, but what's the best technique for this?

press on the BACK of the opposite pad.

saves the pads and the calipers.

so you are pressing on 2 pads which are pressing on a piston.

The Rookie

cooldad wrote:

The Rookie wrote:

Totally the wrong way round, that advances the pads, nothing like bleeding AT ALL.

Presume you are referring to the post before mine?

Yup........

cooldad

The Northern Monkey wrote:

would reccomend using something like a tyre lever, not a screwdriver.

I have a really big screwdriver.

The Rookie

Avid travel spacers are quite good, as are decent table knives, Kitchen knives are a bit of an H and S risk though.......

Chunkers1980

I use a cone spanner.

julianhj

Well, it's happened again.

I've not used the bike this year as I prolapsed a disc in my lower back at the start of the year. I've just got the bike out to give it a service and check that everything is working properly before starting to ride again, and the rear wheel is totally locked, no travel on the lever at all.

Is this a common issue? I've never really got on that well with these brakes.

The Rookie

Sounds like the pistons are sticking just enough to prevent retraction.

With the wheel and pads out, pull the lever gently to push the pistons out, clean round the piston with brake fluid, push back in and repeat.

FishFish

[url=http://www.bikeradar.com/forums/viewtopic.php?p=18655143#p18655143]The Rookie[/url] wrote:

Avid travel spacers are quite good, as are decent table knives, Kitchen knives are a bit of an H and S risk though.......

Agreed with the Avid spacer. I was showing off to my neighbour on the same topic and belted the top of the spacer to maximise the pressure on the pistons. Course it got stuck!

julianhj

I've had a go this morning - the lever is totally rigid, the pads are clamped tight to the disc, no reasonable pressure is getting anything shifted. Time for LBS I think...

Antm81

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19669572#p19669572]julianhj[/url] wrote:

I've had a go this morning - the lever is totally rigid, the pads are clamped tight to the disc, no reasonable pressure is getting anything shifted. Time for LBS I think...

Possibly seals allowing moisture in? My elixirs used to do this, used to have to let a little fluid out of the bleed port on the caliper.

The Rookie

Your money, but I'd try cleaning the pistons first....

dodgy

Sounds like corrosion in the pistons, they're not going back in due to the friction caused by the corrosion.

cooldad

Hardly sudden. Bikes don't like being left unused. Things corrode and seize up.

Rightarmbad

The pads are stuck because of moisture absorbed into the system.
Most will not believe it, but even the brake hose is susceptible to allowing moisture into the system.

The problem you experience even occurs in brand new old stock bicycles when taken out of the box to build.
It has absorbed water and is now overfilled.

Slightly loosen the fill hole and press pistons back until they are fully retracted, tighten up fill cap.
Wrap a rag around the lever near the fill hole to capture extra fluid.
If they are more than a couple of years old then a full flush and bleed is a better option as the fluid now has serious contamination and if a lower level brake may lead to piston or caliper corrosion.

The Rookie

What a load of tosh, how does moisture get into a sealed system? and besides when it does the volume of fluid actually reduces initially, not increases, that myth has been buried several times on here before, stop trying to resurrect the dead!

Antm81

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19675681#p19675681]The Rookie[/url] wrote:

What a load of tosh, how does moisture get into a sealed system? and besides when it does the volume of fluid actually reduces initially, not increases, that myth has been buried several times on here before, stop trying to resurrect the dead!

Apart from the fact it happened with mine every time they were left in a moist environment, but you can't accept anybody else's opinion or accept the fact that something you think can't happen, might happen A sealed system is only as good as the seals it uses.

The Rookie

You haven't shown it was moisture getting in, funnily enough a moist environment is usually hot, also moisture encourages oxidation as well...

If it's not sealed why isn't liquid and the pressure you say is causing the problem leaking out as well?

Antm81

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19675813#p19675813]The Rookie[/url] wrote:

You haven't shown it was moisture getting in, funnily enough a moist environment is usually hot, also moisture encourages oxidation as well...

If it's not sealed why isn't liquid and the pressure you say is causing the problem leaking out as well?

So what would cause it then?

Wouldn't that depend on how much moisture was absorbed and the space it could take up within the system.

Even if it's not moisture it's fluid expansion anyway. How else do you explain letting a small amount of fluid out freeing up the Pistons and allowing them to retract.

The Rookie

Fluid expansion due to heat, these cases are pretty much all reported at the start of summer.

Antm81

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19676560#p19676560]The Rookie[/url] wrote:

Fluid expansion due to heat, these cases are pretty much all reported at the start of summer.

Possible but on my elixirs they'd lock when're bike was kept in the shed in winter but in the house during the same time of year it didn't happen. Surely if it was due to heat the fluid in those would of contracted giving greater lever travel rather than no lever travel and the Pistons pushed out on to the rotors?

In either case the suggested remedy would have the same effect so your argument isn't entirely relevant.

The Rookie

If the system is overfilled due to a poor bleed technique (very common with Avid's) then it can cause all sorts of issues anyway. Organic pads swell when moist for example, so with no 'give' in the hydraulic system due to an overfill that can cause issues.

Rightarmbad

[url=http://www.bikeradar.com/forums/viewtopic.php?p=19675813#p19675813]The Rookie[/url] wrote:

You haven't shown it was moisture getting in, funnily enough a moist environment is usually hot, also moisture encourages oxidation as well...

If it's not sealed why isn't liquid and the pressure you say is causing the problem leaking out as well?

Differing molecule size and the well known fact that dot fluid is hydroscopic, that means that it attracts water.
Try filling a dry system with water and see how the seals hold.

This is a VERY well known effect in both the motobike and automotive world.
Water CAN make it's way into lines through the wall, well proven and the reason why brake fluid bottles use a special plastic to help combat this.
Moisture DOES make it's way past the rectangular seals in the calipers.
Shimano brakes do not suffer from this problem, only DOT systems.

This has not been thoroughly debunked on these forums, only your incorrect and misinformed posts denying that it exists.

Do a bit of research.

The Rookie

Classic fail, Motorbikes (mostly) and cars run open systems and bike srun closed sealed systems, so cars and bikes absorb through the ventilation, this has breen covered on here dozens of times before, also initially absorbing water REDUCES the volume, not increases it, debunking the silly theory you espound only you are chossing to ignore it.