Evening all

After tearing my hair out all day bleeding my brakes repeatedly I think I’ve sussed the spongy back brake issue I know people have (after googling A LOT today).

If your back brake is soft but you are sure you’ve got the best bleed you can, stop bleeding them again and again. Instead, remove the rear wheel and pump the brake lever. Leave the pads in though, so you’re pressing the pads together after a few pumps. Don’t overdo it, it’ll just take 2 or 3 pumps for the piston to push out enough for them to touch and that’s all we want, don’t try to crush them. Now, release the lever and the piston will release a little too. Not enough to get the disc in so jimmy them apart a fraction so you can get your wheel in. Now your brake feels awesome.

I’m going to guess that this is an issue with needing a certain amount of fluid in the caliper body to displace a lodged bubble which, with new pads isn’t always achievable. By removing the spacer or disc you’re allowing additional fluid into the caliper body which you then push out again but you’ve filled the void which you can’t seem to on fresh pads and fresh oil with the spacer or disc. Now that I’ve thought about the issue and typed that, I think I’ll burp the reservoir tomorrow.

Might just be me, but seems to work on my SLX. Takes 2 minutes and no mess. Don’t open any bleed ports as you already know you’ve got a good bleed. Unless maybe you need a little burp.

Hope it helps.

Rick

Question: if your back brake is soft why would you be sure you’ve “got the best bleed you can”?

Sorry, missed a few points.

1) It’s an eBay brake I’ve never used so could be faulty. Got so frustrated I almost bought a new one!

2) no bubbles. Bled 4 times in different ways and literally pure fresh oil going backwards and forwards.

3) straightened out the hose (dropped the caliper off and lever off and let hang, still no bubbles.

4) the main reason I think it’s ok is the pads move from the instant I pull the lever. There’s no initial “slack” per se, the pads just start too far from the disc.

I guess you’re right in that there is a bubble in there but I think it was sitting at the very lowest point of the calliper and I couldn’t dislodge it. Now that I’ve thought about it all I think I’ve done is move an awkward bubble elsewhere and it’s having no (or significantly less) effect. I’ll let it rise and just burp the reservoir slightly in the morning.

Open to suggestions and it might not have worked and just temporarily feels better.

I do the same on Guides but it’s to get the lever feeling nice after the pads have worn a fair bit.

On an actual imperfect bleed (can’t speak for other makes here) I use the rubber band on the lever overnight method which can help a lot.

1. This happened because the brakes weren’t properly bled and there was an air bubble in the system.

2. The bubble wasn’t in the caliper, it was in the hose, up near the lever. What you have done is pumped fluid into the caliper, then forced it back into the lever. This carried the air bubble back into the reservoir at the lever. If the bubble was in the caliper, it would still be in the system. It doesn’t matter if it’s in the caliper or the hose, if there’s air in the system, it will feel spongy.

You may well have just freed up the caliper pistons that were sticking to their seals and being pulled too far back when you released the lever, it’s something I do with most new brakes, and often makes a huge difference to lever feel. Hope cover the process in their caliper centering video, and quite often you’ll feel a pop through the lever as the pistons release.

For years now I’ve been bleeding my Shimano brakes by removing the caliper, pumping the lever to get the pads to kiss, bleeding the system upwards, closing the system, prying the pads apart just enough to get the disc in, then riding.

Perfect, solid feel every time. 👌

If you’re bleeding it with the pistons extended, you need to push them back in with the reservoir open to make sure the system isn’t overfilled. If you overfill it, it can make it impossible to fit new pads, even if it works fine with worn pads.

wot coatsy says – what you have done is reset sticky pistons

Thanks for the replies.

What’s the best advice here then? I was searching for an answer yesterday and there wasn’t many good ones so we should help future struggling shimano owners.
Is it “if you’re sure you’ve got a perfect bleed but the lever is spongy, then you haven’t got a perfect bleed, and if your tried multiple times there is probably a bubble trapped somewhere you aren’t moving with you syringes. Try pumping the lever without the disc in between to make the pads kiss. Then push the pistons back manually (after opening the reservoir?) and your brakes should feel nice. Then allow the bubble to work to the top and perform a “5 minute bleed” just at the reservoir side by manipulating the lever.

Is that right?

Coasty/TJAgain – piston stuck to seal makes sense. In that case, do I need to burp them again? If they are sticking to seals then does that mean there is it is not air in the system?

You can do the same thing without the pads in. Using 5or6 links of chain in a loop between the pistons. Saves pads from possible contamination and use for those excess links when you change chains.

Sticky seals is nothing to do with bleeding. Now you have freed them they should be fine. a bit of mineral oil on the pistons might help if you work them in and out a few times but be wary of pumping them out too far and of chipping them

A sticky piston will still give a firm lever feel, just excessive travel before the pad contacts the rotor. If they’re actually spongy, that will be air in the system, which means they haven’t been bled properly.

Thanks guys. I had excessive travel but they did bite in the end if I removed the lever from the bars to get enough distance to pull tight.

Sounds like it was a piston issue. Still firm this morning. Looks like that is all done now then. Result!

This is the only technique you need to follow to get sorted with shimano brakes.

I look after a fleet of bikes with cheap shimano brakes and this method is the easiest way to get them back to original working order.