Anyone know anything about disc brakes?

Hadn't thought of that actually, I had them replaced at a bike shop a while back with some other stuff and didn't specify

Too many variables; if you get a lot of shit on the pads, they can wear VERY fast with some organic compound pads. Metallic/semi (as per @andyp ) will help but ultimately, there's no hard and fast rule. I tend to have a spare set for my bikes and whenever I change, buy more so there's always at least one set available.

I've just replaced a 4 year old set of ultegra discs which have seen maybe 20k km and are still "okay" but they're going on a less used set of wheels now.

Anyone got any tips for maintaining and setting up Hope V4s?

Constantly find they need fettling with, and that they're incredibly hard to get evenly centred on the rotor. Pistons often drag too.

(Currently finding that I can't adjust the caliper inboard enough to prevent rubbing)

Bin them and buy any other brand…. Sorry.

Doing a piston advancement service can help, you can buy little 3d printed spacers that allows you to advance one piston at a time to give them a clean and help loosen them up. It can be a bit tedious, takes a bit of time to do all 8 pistons but it will definitely help.
The pistons might not be retracting fully and causing rub.
The process sounds worse than it is, it’s fairly simple but just takes a while.
There’s a couple clips on YouTube that show a run through the process.

Anyone tried the new 12 speed shimano calipers and have anything to say about rub based on their claims of 10% wider pad clearance? Thinking of replacing R7020 with R8170 calipers.

Rubbing may also be my fault for using thicker SRAM rotors (1.8mm vs 1.75mm) and should probably start with those but who doesn't like a project..

If they're getting on a bit, the pistons get scuffed up and stick. Doing a full service on them, and replacing the phenolic pistons with the lovely shiny new stainless hybrid ones is worth a shot. They're smoother, easier to set up, and despite actually retracting less, also seem to rub less as they retract consistently

Unless you're already using the new V4s with the steel pistons, in which case yes, they do need balancing every now and then

Because I'm an idiot who does everything last minute, I had the displeasure of finding out that both my hydraulic brakes were completely ineffective on the start day for PBP.

This was after the bike had been left on my home trainer for ~2 months. The brakes were squealing if I squeezed the levers to hard, and could only slow me down, at best. The rear brake started functioning almost properly again after ~20 km, but the front brake only got slightly better over the course of PBP, and is still squealing.

This is the second time this happens, on different bikes (first bike had GRX Di2, this one has 105 levers with a mix of calipers, 105 in the front, BR-RS785 in the back) in different apartments. I was running resin pads both times though. I don't think it's a bleeding issue, as the levers still feel relatively stiff.

1°) Does anyone have any idea why this happens?
2°) Is there any way to fix it now without changing the rotors and the pads?
3°) How can I fix it before going on a ride? Or how can I make sure it doesn't happen when I take the bike of the trainer before putting the wheel back in (e.g. clean the rotor with isopropyl alcohol maybe?)?

Sounds like they're contaminated, this could be from you or there's a chance of a leaky piston if you're sure it's not from you. You can try embracing the squeal and get it really hot braking down big hills but it's an unlikely and temporary solution, sorted my front out for a ride though. Sanding and cleaning the pads and rotor may help, otherwise it's clean rotor and new pads and if it returns after being really careful, new caliper.

I haven't had to do this when changing pads, but did need to shim out a Spyre caliper with 1 or 2 thin washers when swapping between forks once (didn't die). Though I'd say as long as the pad is still making contact up to the outside edge of the rotor, leave it be. Maybe chamfer the bottom edge of the pad with sandpaper if the noise is too annoying?

Sorry for the late reply! Yes, it does sound like they're contaminated, but I have no clue how it could have happened.

Can dust contaminate pads? That's the only theory I have given the circumstances. Both times this happened, my bikes were sitting in my living room in a dusty and humid apartment. If I have to replace a 40€ set of pads every time my bike sits on the trainer for more than a month, it's gonna become expensive really quickly. A rim brake frame seems tempting but alas I need 30mm tyres and mudguards.

It is not a leaky piston issue as both front and rear got the same issue, though I'll double-check that there's no oil on the calipers.

For cleaning, do you think isopropyl alcohol on the rotors will suffice? I'm worried I might not clean the rotors well enough and they might re-contaminate the new pads.

do you think isopropyl alcohol on the rotors will suffice?

Yeah, but also sand them between IPA cleans, I spin the wheel and hold sandpaper/emery cloth to them as they spin, watch your fingers.

Something in your home has got on them. Dust / some kind of cleaning fluid / lubricant of some kind.

Few of these things can survive the temperature generated by a steep descent with brakes on full whack. Repeat this until they are smoking.

FWIW, Shimano brakes are shite, in that the pistons have a habit of fucking themselves and allowing small but ruinous amounts of fluid to escape. It could simply be both being fucked.

Grease vapour from cooking is an effective contaminant.

Polish containing silicone, …

Polish containing silicone

What does Anoushka have to do with it?

The only thing that's happening in this living room is vacuuming, and a fan the diameter of a bike wheel blowing on the bike when I'm on the trainer. Guess I will have to sew some disc caliper socks to put on them while the bike is on the trainer...

Thanks @Howard, I will check the back of the pads for fluid thoroughly, though the rear caliper has less than 3k km on it. What's the good option when you want to use mineral oil, Hope calipers?

Get new Shimano callipers then warranty them when they inevitably do it again.

🥲

I want to tell you it's not that common, but I did order myself a new caliper the other day as I suspect it's leaking. Although it's lasted about 3 years and I've had loads of other shimano brakes never have an issue, including a zee that's been smashed with balls and mallets repeatedly and always been perfect. I've replaced a few through work but I've seen a lot more not need replacing.

that's been smashed with balls

Have you considered cycle training?

If I can smash my disc caliper with balls I obviously don't need it.

I have a pair of Shimano disc brakes that haven't been used for ~4 years, and appear to have lost their bite.

Have cleaned the pads/rotors and still no luck.

No obvious fluid, so I don't think they're leaking, and there doesn't seem to be any sponge-yness so I don't think they need a bleed - the leavers don't bottom out on the bars.

My gut feeling is that they just need topping up with fluid, but I'm already at the limit of my knowledge, and don't have the kit to try.

Also, if I'm realistically only going to ride it a handful of times before next spring, is it worth just hanging it up again and getting it serviced then, rather than now and then leaving it unridden for another 6 months

There are loads of posts on other forums about Shimano pistons weeping onto pads if left unused for long periods of time - wouldn't be surprised if that's happened here. I'd clean rotors again really thoroughly with isopropyl, clean the calipers after bleeding, work the pistons a bit to get them moving freely, then replace the pads and bed in again. Shimano bleed kits are about £15, if you're not totally cack-handed it's a surprisingly simple job to bleed them

I’ve just ordered a set of these https://www.ribblecycles.co.uk/shimano-disc-brakeset-j-kit-bk-bl-mt201r-br-ur300f-950mm/ for the flat bar conversion on my Sequoia. The front hose is routed through the fork leg, can I just disconnect the hose and use the existing one on the bike? It’s not the end of the world if I have to run the hose externally with some cable ties to secure. It’s only for work and a bit of quaxing.
Should have said they are rs505(?) 105 road at the minute.
Ta