One brake piston coming out futher

Shimano M7000 brakes and no matter what I do, the in knee piston always comes out more than the outer.

The outer seems to get stuck after each ride.

I’ve cleaned them, worked the free and on the bench they are great. 2 rides later and the outer is only half way out.

When the pads are worn the levers touch the bars.

I’m also having to burp the brakes every ride or air is in the sytem, despite fresh bleeds and.

Have I hit serviceable life?

I had a pair of Hope brakes like this. Eventually got them working nicely but they seemed fussier than any others I’ve had.

More servicing needed? If they’re getting squishy then check for leaks  maybe it’s time for new seals and hoses?

reaching the bars when the pads wear – do you have enough fluid in there? Asking as I thought I had enough fluid in one shimano brake until the pads wore and then realised I was mistaken

Push both pistons in fully and rebleed the system from the caliper.

Then if still no better try holding the good piston with a tyre lever and push the brake lever. Move the bad piston out slightly then fit the pads. See if that works?

1) Grease the pistons with Castrol red rubber grease.

2) Unless you’ve got braking issues, it doesn’t matter and will always happen. Look at the piston. The fluid comes in one side first. That piston will push more when the wheel is out. When you’re hammering on the brakes, once that piston has touched the rotor and therefore has some resistance, the other one will come out and meet it. Unless when the wheel is in the rotor bends on brake application, you’re fussing over nowt.

https://www.pinkbike.com/news/tech-spotlight-brake-caliper-piston-service-assessment-2014.html

Interest article. I am currently wrestling with an old Shim brake that appears to be fine- plenty of lever pressure, both pistons move, pads have plenty of meat, but doesn’t do its actual job- stop the bike.
I shall have another look and try to balance pistons more accurately

The fluid comes in one side first. That piston will push more when the wheel is out. When you’re hammering on the brakes, once that piston has touched the rotor and therefore has some resistance, the other one will come out and meet it.

Both sides will have exactly the same pressure. The difference is caused by one seal being slightly tighter than the other. It can be on either side. As long as they center properly once the wheel is fitted, it makes no difference.

I mean sure thols, either way – the result is the same. It doesn’t really matter as the seal friction is unlikely to be more significant than the pressure of pad on rotor, unless you’re not getting good enough braking performance.

The fluid comes in one side first. That piston will push more when the wheel is out. When you’re hammering on the brakes, once that piston has touched the rotor and therefore has some resistance, the other one will come out and meet it.

Thats how car and motorcycle discs work. However in a bicycle once the first pad has hit the disc it will continue to travel bending the disc as the disc is so flimsy. the result is once both pads contact the disc is pushed off centre. this is the main cause of spongy brakes and long lever travel.

this is why Hope rely on a better method of centralising. I think this effect is worse with hope – tighter seals?

The pistons don’t slip through the seals in normal use. The seal flexes to allow the piston to move, and the seal flexing back is what retracts the pads. Only when the limit of flex is reached does it slip through a tiny amount, that’s how the self adjustment works.

One seal may be stiffer than the other, it might’ve been from a different batch at the seal factory or something best option would be to replace the seals, but I don’t think you can do that with Shimano. If it’s only slight, ignore it.

As Molgrips says. Plus if one piston is a tighter fit then that piston is less able to adjust for pad wear so the brake performance deteriorates over time

Each time I change pads I always pump the pistons out a bit and spray silicon on them, then work them in and out a bit. Clean with isopropyl alcohol before fitting tha pads.

Have done this with Avid/SRAM, Shimano and Formula in the past and it let them all running well. Until the Formula MC seals died and the Shimano caliper leaked anyway!

I had this with an SLX brake after about nine months. Bled it (Syndicate/Marshy’s method), pushed in pistons having removed pads, pumped them out a bit, lubed with silicone grease on a cotton bud, pushed back in; centred caliper by eye relative to the rotor, still without pads; installed pads, pulled lever full-on, held tight while slackening off then retightening caliper mount bolts. Job done, still using the brake three years in.

Does silicon spray play well with Shimano mineral oil?

Silicone doesn’t hurt anything seal-wise, Shimano or otherwise

some thoughts on disc brakes

https://cyclingtips.com/2021/02/chris-froome-is-mostly-right-about-disc-brakes/

I think most of those criticisms of road discs are exaggerated or irrelevant for the average user. Keep in mind, I don’t ride road so their priorities will be different to a recreational mountain biker.

If you’re racing at the very elite level, maybe a couple of hundred grams is worth worrying about, but most riders are better off having a fractionally heavier bike with brakes that work consistently. As I understand it, there is a minimum weight for road bikes and the pro teams can build bikes below that weight. If that is the case, then the weight issue is irrelevant.

Obviously, the pro riders descend very steep mountains very fast and their brakes have to dissipate a lot of heat. If their brakes are fading due to overheating, they need to look at rotor size, pad material, finned calipers and pads, etc. It’s possible to make disk brakes that dissipate massive amounts of heat, but if you’re more concerned about saving 50 grams than braking performance, maybe you need to reconsider your priorities.

The rubbing thing is mostly exaggerated IME. You sometimes get a very slight rub, but that’s not the same as a dragging brake. I’ve been using the same 8″ rotors on my bikes for years and they don’t rub at all, so rubbing isn’t inevitible.

The pistons not extending identically is another distraction. This makes no difference to brake performance. Even with properly serviced brakes, one pad will always contact the rotor first, then the other pad will contact and then they will apply the same clamping force to the rotor. If the rotor is being deflected, then the brakes need servicing because one piston is stuck.

The reliability/servicing thing is nonsense. Disk brakes work with minimal maintenance for years. If bleeding brakes and changing pads is too difficult, maybe they need to find a new mechanic.

Fifteen or twenty years ago, this forum was full of threads about how suspension and disk brakes on MTBs were unnecessary, too heavy, too difficult to service, etc. Then it was dropper posts. Once people get used to the new technology, they get used to the benefits and it’s hard to imagine anyone wanting to go back to brakes that don’t work when it rains.

I’ve got a few bikes, and the least troublesome brakes seem to be the ones used most frequently, with lots of hard braking to get them hot and burn/scrape any contaminants off.

Same with the car as well. Leave it for ages and it ends up with rusty discs and a stuck caliper.