Disc brake binding issue

[Scarletpimpofnel]

My friend's ebike has Shimano disc brakes front and rear. One of them has been binding making it hard work pedalling.

Today I took the wheels off and replaced the brake pads. With the pads out I gave the top of the pistons a clean as best I could. As far as I could tell the pistons have a rubber gaiter around them between them and the cylinder (but it was very dark in there and hard to see). The spring clips (Shimano) to hold the pads apart and presumably return the pistons fully home seem very puny to me, next to now power behind them.

Anyway after cleaning I put the old pads in then forced the pistons home with a screwdriver then got my friend to apply the brake multiple times and each time I pushed the pistons home again, sometimes I'd do each pad individually whilst holding the other pad "home" so I could check both pistons were moving freely. After doing this many times I was happy that all pistons were moving freely and returned home freely.

Anyway still she is complaining of some binding of the front brake. The discs look unworn to me but in anycase the piston should move out the way of an untrue or worn disc I assume?

The pads are resin pads if that makes any difference but the binding was there with whatever was there before. The bike has done about 3000 miles in mainly dry weather but the face of the brake pistons had light rust on.

Is there anything else I can or should have done? TY

Edited by Scarletpimpofnel on Friday 17th February 17:06

[_Hoppers]

My MTB disc brakes started to bind quite heavily after not using it for some time. The brake fluid had absorbed moisture causing expansion in the system, I let out a bit of fluid from the bleed screw on the caliper and it freed up just fine.

[Scarletpimpofnel]

_Hoppers said:

My MTB disc brakes started to bind quite heavily after not using it for some time. The brake fluid had absorbed moisture causing expansion in the system, I let out a bit of fluid from the bleed screw on the caliper and it freed up just fine.

Thanks. Is this just a case of unscrewing the screw a little and squeezing the lever a little to squeeze out a small amount of fluid? Is that the right way to do it? TY

[_Hoppers]

Scarletpimpofnel said:

Thanks. Is this just a case of unscrewing the screw a little and squeezing the lever a little to squeeze out a small amount of fluid? Is that the right way to do it? TY

When I (slowly) unscrewed the bleed screw the fluid came out by itself as there was a bit of pressure there. I you try this don't let too much out, there could be another reason for the binding? If nothing comes out initially pulling the lever a little should bleed some out.

[OutInTheShed]

The spring clips don't return the pistons.
The pistons are pulled back slightly by the seals.
Fraction of a millimetre, that's all.

The pad clips stop the pads wobbling and damp squealing.

You should see each piston move when you move the lever.

Corrosion of the caliper pistons is quite likely.
A warped disc is a possibility.
If the brake is dragging the disc will get hot and warp, so it gets worse.

A new disc is quite cheap.

I had a lot of problems with the back brake of my old mountain bike.
I just changed the whole system for not much money.
Master cylinder and caliper came already plumbed, filled and bled.

Simply moving the pistons a few times may free them up, otherwise a total strip and clean may be needed.
Note that some bike brakes use oil, not brake fluid.

Also check the pad moves freely in the caliper, no corroded edges.
The pad isn't delaminating? Warped?

Check the caliper is aligned to the disc, centred etc
Check the wheel is correctly installed in the fork!

It's like the early days of disc brakes on small motorbikes circa 1975....

[Scarletpimpofnel]

@Outintheshed - Ok lot's of good points there. The pads that I replaced had all worn evenly. As in thickness the same on all and even wear across the face of the discs. I'll have a look at each point you mention but I don't have dial test indicators etc to test for warp etc so may need to refer it to someone who knows what they are doing ! I'll try the releasing fluid pressure first as that seems easy and zero cost. TY

[IroningMan]

If there's enough resistance that the rider can actually feel it, as opposed to just hearing an annoying noise, then I'd expect a fair bit of heat to be generated on a short test ride. Enough to feel with the back of your hand, anyway.

What kind of condition are the wheel bearings in?

[Lotobear]

You don't need a DTI to check if a MTB disc has warped - just do it by eye.

I've succesfully trued a number of dics by simply putting them in the vice and tweaking by hand - approximately true is enough as they are bendy things unlike a car.

I would pump the pistons in and out a few times a lube them, usually enough IME to sort sticking brakes. If not do a bleed.

[OutInTheShed]

Yes a binding brake will get hot.

I thought 'god I've got unfit', then I burned my finger on the disc.

If its a plain bolt on disc, a good way to check for flatness is to put the disc against a mirror, I think any warping that matters would be obvious. I use this method to check clutch plates, with a feeler gauge.

I agree, very often just moving the pistons repeatedly will free them up.
Sometimes new pads will move the piston to a happier area in the bore.

[Scarletpimpofnel]

IroningMan said:

If there's enough resistance that the rider can actually feel it, as opposed to just hearing an annoying noise, then I'd expect a fair bit of heat to be generated on a short test ride. Enough to feel with the back of your hand, anyway.

What kind of condition are the wheel bearings in?

The bike is 18 months old and done 3000 miles so I would hope they are in good condition (bike is a Cube Pro Touring ebike). But that said I had no idea nowadays chains have to be replaced every 1000 miles and brake pads every 750 miles. My last biking experience was 40 years ago when the chain lasted forever and the brake blocks were rock hard rubber that also lasted forever. Everything seems disposable nowadays, so maybe wheel bearings only last a few hundred miles too !

[IroningMan]

Scarletpimpofnel said:

The bike is 18 months old and done 3000 miles so I would hope they are in good condition (bike is a Cube Pro Touring ebike). But that said I had no idea nowadays chains have to be replaced every 1000 miles and brake pads every 750 miles. My last biking experience was 40 years ago when the chain lasted forever and the brake blocks were rock hard rubber that also lasted forever. Everything seems disposable nowadays, so maybe wheel bearings only last a few hundred miles too !

They should be fine, but equally if the wet’s got in then one might have rusted. I was just thinking that a seized wheel bearing would be more likely than a rubbing brake to cause enough resistance for the rider to feel it.

[aterribleusername]

Hard to tell without seeing the brake but it sounds like it's not centralised correctly. Shimano brakes are annoying for compensating for this well but it leads to one pad forcing the other side to over compensate and that leads to a bit of drag but not enough to show up as uneven wear (the draggy pad has less force through it when you brake so the wear is more on the other pad, the wear from the drag can get balanced out so they look even!). There's also the possibility that the old fluid (it'll be mineral not DOT) is breaking down and expanding slightly meaning the pads don't retract enough, easily fixed with a bleed. If the old fluid is black rather than red then that's one issue solved.

I've always used the Hope method of centralising and balancing rather than the Shimano method (I have both brands on different bikes) and it really does make a difference.