Disc brake frame facing

chaymck

I'm having issues with the rear disc brake on a aluminium road frame. Through axle. R8000 series. Flat mount.

The braking performance is crap. And the pads bearly slow the rotor.

The bleed is good.

I've tried new pads and rotors and that is OK for a ride but performance drops off quickly.

I've tried a new caliper and even a new wheel. No improvement.

LBS have checked everything and suggested as a last resort facing of the frame mounts. (They don't have the tool, so i will need to go elsewhere).

Bike is out of warranty so can't be returned.

Anyone had success with facing. Or any other suggestions?

Thanks

maringirl

Park Tool do a very fancy looking tool the DT 5.2 plus adapters for doing flat mounts. It costs around £350 so not one u are gonna buy for what will be one-off use! So yuo need to find a shop that has one - good luck!

veronese68

Sounds to me like the LBS don't know what to do and are clutching at straws. Why would the brake improve for a ride with new pads and then go off again if facing the frame is the issue? That makes no sense at all, if facing the frame was the issue new pads would make no difference.
That sounds like it could be an issue with contamination if new pads and rotors start ok then go off. You say the bleed is good, but is it possible a piston is cracked and hydraulic fluid is getting onto the pad surface? Do you drag the back brake gently? I had a tendency to do this and on one bike I noticed performance of the back brake would deteriorate slowly. I started to use the back brake a bit harder every now and then which seemed to help stop the pads glazing and brake performance improved. Try getting up to speed and just using the back brake harder a few times in succession to slow you down, but not to a complete stop. And obviously avoid locking up. It might help and it costs you nothing to try it.

First.Aspect

This. Facing isn't the issue. The pads would bed in to compensate and so if anything the braking would get a bit better over time.

Glazing of the pads is easy to fix, if that's the cause. Take your old pads out, rub them face down on a flat bit of concrete and put them back in again. Chances are that if that makes little difference, that's not the cause.

Shimano calipers are known to leak occasionally, but you say you've tried a different caliper, and your posts suggests you have issues with the braking per-se.

If it really is multiple calipers (i.e. both original ones and a replacement) then I'd be looking again at things like where the bike is stored, how you are cleaning it and even whether you do actually have air in the system. If the LBS that has done the brake bleeding is suggesting a costly solution that it is unlikely to make a bind bit of difference, I'd be questioning everything.

chaymck

Facing is £35 quid at another lbs.

The bike is stored along with other bikes. Which don't suffer in the same way.

So unlikely to be me, dragging, or storage location.

I'll give it a go and report back.

First.Aspect

Requests advice on X.

Advice is don't bother with X.

Does X.

DeVlaeminck

No expert but did you change pads/clean rotor after changing the caliper? Just thinking if the old caliper was leaking it will have contaminated the pads and they'll still be contaminated even if the new caliper isn't leaking.

chaymck

First.Aspect said:

Requests advice on X.

Advice is don't bother with X.

Does X.

Suggested fixes were for things I've tried.

No new ideas.

Thanks for your input.

chaymck

DeVlaeminck said:

No expert but did you change pads/clean rotor after changing the caliper? Just thinking if the old rotor was leaking it will have contaminated the pads and they'll still be contaminated even if the new caliper isn't leaking.

Yes. New rotor and pads with new caliper.

First.Aspect

Uh huh.

imposter2.0

Take off and then nuke the bike from orbit. It's the only answer...

pilot_pete

What is the lever feel like? Does it firm up as you pull and hold the lever, or does is feel spongy even when pulled fully on? How far does the lever travel? Are the levers the same series as the calipers? What rotors are fitted? Are they part of the same series, or another brand altogether? What brand pads and what material are fitted? What hoses and fitting kit were used? Has it always been like this since new?

Sounds to me like if the components are all compatible and correctly fitted it is more likely a pad contamination or a bleed issue. You need to make sure there is absolutely no trace of fluid anywhere from the lever, unions, hose, caliper and pistons. A cracked piston (easily done with ceramic pistons and a ham fisted mechanic) is one possibility as are lose hose connectors or even over tightened hose connectors leading to a badly crushed, out of shape olive that will not seal.

Assuming you have no leaks, and the pads are not contaminated, I would then re-bleed the brake by simply flushing through with some new fluid. Make sure you get all the air bubbles out of the caliper and the lever reservoir. Look up how to do it properly. Make sure you get a firm lever feel once done. If the lever feel remains spongy or goes all the way to the bars you still have air trapped or a faulty component. It can be harder to remove all the air from a rear brake due to the long hose run, which can also have ‘uphill’ sections with the bike standing on its wheels. Bleed it with the front wheel elevated significantly in a stand to ensure the caliper is at the lowest point and the hose runs uphill all the way to the lever. Even the tiniest bubbles can cause sponginess over the long hose run. Pulling the lever on hard and using a strong rubber band to hold it there overnight can help force reluctant air bubbles up to the reservoir if all else fails.

If this doesn’t sort the problem and the lever feel doesn’t get firm I would be suspecting a problem with the lever. It’s the only bit you haven’t considered yet.

Charlie_Croker

^ As above
But don't forget to "de-gas" the new fluid before pumping it into your system
It can be quite surprising how much you get in fresh fluid and it does make a difference

trevor.hall12

If the bike is now out of warranty then its been OK for years ,that would also suggest the caliper mounting is OK.
Because of the short term improvement with new parts it sounds like a leak or bleed issue

First.Aspect

It's his £35 and time I suppose. And I sense that will need to be ruled out before OP returns to things that he's already ruled out.

We could all be wrong, I suppose.

chaymck

So. Facing was fine. But a replacement of the brake hose has sorted the issue.

veronese68

Glad it's sorted. Not surprised that facing wasn't the issue as that was obvious. I am surprised changing the brake hose has fixed it if there were no issues with the bleed.
Sounds like you found a better bike shop for fixing it at any rate.

edward.s

I've seen this where a hose has got crimped. Pleased you got to the bottom of it.