Mystery rear disc brake problem.

[Andy in Germany]

Elder Son is having a mysterious problem with the rear brake on his Canyon Neuron. Repeated attempts to change the disk, pads and oil have failed to make it work and at the moment it is not responding when the lever is pulled, Attention from a bike mechanic (Elder Son) has been fruitless, and he's probably going to take it to someone with more experience.
Has anyone else had this problem with discs?

 

[Darius_Jedburgh]

The pistons do get stuck.
Operate the lever with no wheel/disc in place.
Pistons might only work on one side - one stays static and that's normal, but newer models have both pistons moving.
If both move then clean them, especially the edges, when fully out. Brake fluid on a cotton bud works well.
If only one moves then clean that similarly.
Push back and operate. Maybe repeat.

Obviously remove pads before starting. (Sorry for stating the obvious, but some folk....)

You may lose some fluid and have to rebleed.

 

[fossyant]

Disks should be fine - even if contaminated with oil a good scrub will clean them. Pads need to be replaced if contaminated.

Is it leaking oil ? You could have a jammed piston in either the caliper or lever.

What's happening at the lever end - is it moving properly. I ask as SRAM Guide ettc had an early issue with the piston in the lever - they expended just enough to jam - usually in hot weather, but it's something that would get worse. I replaced mine with the revised piston.

If you pop the pads out, carefully squeeze the lever (assuming the lever is working - see above) and see if it pushes the pistons out. If it doesn't you've got a problem in the pistons (be careful as you don't want them popping out if working.

Have they been bled properly ?

 

[Jody]

What brakes are they and what is happening with the lever when pulled?

Also what prompted them to be looked at. Was it a pad change or had a problem started to develope?

 

[Andy in Germany]

What brakes are they and what is happening with the lever when pulled?

Also what prompted them to be looked at. Was it a pad change or had a problem started to develope?

They're Shimano SLX. I'll know more when the Elder Son turns up.

 

[Jody]

They're Shimano SLX. I'll know more when the Elder Son turns up.

Easy ish brake to blade if you have the correct tools. Make sure he used mineral oil instead of brake fluid.

 

[si_c]

Is the rear caliper leaking oil? If so could be failed pistons so new caliper time.

 

[Andy in Germany]

Just asked, the problem started on the original brakes, which were "BA 200 or possible BT 200". The attempted repairs went through all the remedies described above and eventually extended to a complete replacement with the SLX's. Oddly, the problem persists.

 

[Jenkins]

Is there a blockage or kink in the hydraulic fluid line?

 

[Andy in Germany]

Is there a blockage or kink in the hydraulic fluid line?

Everything has been replaced including the brake line; the only thing not replaced is the disks themselves.

 

[Jody]

Everything has been replaced including the brake line; the only thing not replaced is the disks themselves.

As long as he's getting a good lever that has pumped up then it sounds like the disc has contaminated the pads.

 

[Andy in Germany]

As long as he's getting a good lever that has pumped up then it sounds like the disc has contaminated the pads.

How can that happen, and how could we solve the problem?

 

[Jody]

How can that happen, and how could we solve the problem?

Leak from the old caliper. Assuming I understand you right and he's changed the full brake set (lever, caliper, line and pad).

Either clean or replace the disc and replace the pads if the brakes are working right. Just make sure the pads are moving freely and the lever is firm so you've ruled out seized pistons and bad bleeds.

 

[Andy in Germany]

Leak from the old caliper. Assuming I understand you right and he's changed the full brake set (lever, caliper, line and pad).

Either clean or replace the disc and replace the pads if the brakes are working right. Just make sure the pads are moving freely and the lever is firm so you've ruled out seized pistons and bad bleeds.

Yup, that was correct. So the leaked fluid is still sitting on the brake disc? I'm guessing something like cleaning alcohol followed by warm soapy water should shift that, is that safe?

 

[Sterlo]

Just the alcohol will do fine on the disc. Contaminated pads may be saveable if not too bad. Clean with alcohol, lightly sand the surface then do with alcohol again and should be okay.