Front brake issues

[LTPC]

Does anyone have any idea why my bandit 250’s front brake feels rock solid when I’ve bleed it off the bike with pad spaces fitted and then goes soft when fitted back on the disc?

It’s as though the lever travel increases once the caliper is on the disc. I’ve tried bleeding it on the disc and it makes no difference either. When I bleed it off the disc and it’s firm I have the pads spaced the same width they’d be when sitting on the disc.

Any ideas what to try next?

[OddDuck]

It's possible to have squash in the hydraulic system via air bubbles, or in the mechanical system where pads are pushed onto disc. Is the disc perfectly flat / new or has it got a few k's on it? If there's a raised edge on the contact area (which the pads can sit on now that they've been reinstalled) that can do what you describe. You'll be feeling spring flex through the pad body. Same thing can go for wave lines on the disc face, if it's a drilled rotor and has been wearing down.

Another possibility is stiction in caliper body slider pins, if your bike uses these and they have jammed then the disc will be flexing sideways under the piston instead of the brake caliper centering properly. I'm not familiar with the brake setup on your bike so let me know if this is incorrect.

[LTPC]

It's possible to have squash in the hydraulic system via air bubbles, or in the mechanical system where pads are pushed onto disc. Is the disc perfectly flat / new or has it got a few k's on it? If there's a raised edge on the contact area (which the pads can sit on now that they've been reinstalled) that can do what you describe. You'll be feeling spring flex through the pad body. Same thing can go for wave lines on the disc face, if it's a drilled rotor and has been wearing down.

Another possibility is stiction in caliper body slider pins, if your bike uses these and they have jammed then the disc will be flexing sideways under the piston instead of the brake caliper centering properly. I'm not familiar with the brake setup on your bike so let me know if this is incorrect.


It's a single floating caliper with two pistons. It could be the pins as it is a floating caliper but why wouldn't it stick when the caliper is off the disc? It all started with when I put new pads in. Before that it's always had a nice firm lever.

[OddDuck]

It's a single floating caliper with two pistons. It could be the pins as it is a floating caliper but why wouldn't it stick when the caliper is off the disc? It all started with when I put new pads in. Before that it's always had a nice firm lever.

Sounds like mismatch between new pads and used disc surface, then. It does not take much flex to give this effect. The bright side is that it goes away with use and the brake firms up again, once the pads bed in and match the disc, you'll be OK.

[F5 Dave]

So when you hold the disc or caliper what is moving or flexing?
The pins are replaceable as is the pad retaining pin.

[LTPC]

Sounds like mismatch between new pads and used disc surface, then. It does not take much flex to give this effect. The bright side is that it goes away with use and the brake firms up again, once the pads bed in and match the disc, you'll be OK.

Fingers crossed that's it. Will put some kms on the new pads and see how it goes.

[LTPC]

So when you hold the disc or caliper what is moving or flexing?
The pins are replaceable as is the pad retaining pin.

To be honest there's not a lot of movement. The disc flexes as I pull the lever and there's slight movement of the caliper on the pins. It's all pretty tight given the new pads which is why I don't get what's causing the excess lever travel.

Will bed them in a little and see if that helps.

[Cadbury]

To be honest there's not a lot of movement. The disc flexes as I pull the lever and there's slight movement of the caliper on the pins. It's all pretty tight given the new pads which is why I don't get what's causing the excess lever travel.

Will bed them in a little and see if that helps.

The disc should not flex at all, the caliper should slide and self-centre on the disc. Sounds to me like the piston is pushing the pad onto the disc but the fixed far side pad is not being pulled on. How clean/lubed are the pins and are the caliper piston seals clean?

[OddDuck]

The disc should not flex at all, the caliper should slide and self-centre on the disc. Sounds to me like the piston is pushing the pad onto the disc but the fixed far side pad is not being pulled on. How clean/lubed are the pins and are the caliper piston seals clean?

+1, suggest pulling the pins and cleaning / relubing everything and trying again. I didn't know you were seeing disc flex earlier, that's probably most of the slack in the system that you're seeing now.

Apologies if I'm telling you something you already know... there's a special grease for brake caliper pins which is sold in little sachets at outfits like Repco or Super Cheap Auto, like this:

http://www.supercheapauto.co.nz/Product/Permatex-Ultra-Disc-Brake-Caliper-Lube-14g/113948

Normal grease can attack brake system rubber seals, also it can do things like spread and contaminate brake pads.

[F5 Dave]

Disc should not move. Pins are replaceable but you might have to do research into whatever models have the same bits