Pushing back in disc brake pistons

[Chik]

The pads in my rear DA R9170 caliper needed replacing. The pistons needed to be pushed back in to make room for the fresh pads. However, despite considerable effort using Abbey's Stu stick, I was not able to push them back far enough to accommodate the new pads.

In the end, I moved the front pads to the rear and installed the new pads in the front because I was able to push the pistons in the front caliper far enough.

Is there a trick to pushing the pistons all the way back in? What do you suppose I'm doing wrong?

Also, I noticed that the bolt fixing pin is missing from the rear caliper. I don't see any issues with the bolt being properly fixed in there. Should I be worried? I suppose I should try to get a replacement pin in any case...

Thanks!

[andrew flowers]

Use a plastic tire lever without the pads in the caliper, but be careful. If the piston is crooked it can crack. I use pedros ones because I like the colours they come in

The pins fall out all the time. Shimano doesn’t list then as a normal replacement part. If you are worried run the cotter pin you bend. I’ve seen the retention rod fall out, and it is really bad.

When swapping your pads articulate the pistons and make sure they move properly. My procedure is as follows: have the caliper loose on the frame. Put the bleed cup on the lever. Reset the pistons without pads. Put the pads in, and put the wheel in the frame. Pull the lever a bunch to get any air out. Check the caliper. If one piston has moved more than the other, do a caliper service. Line the brake up and off you go.

To bed your pads faster, you can do this: lightly sand them with clean emery cloth (just rough them up). Run them under water, then rub them together so they get kind of gooey. Pat them clean on a clean cloth, and install. They will grab within a few pulls of the lever.

[bigbill]

I used the plastisol coated end of a Park cone wrench to push mine open.

[dashDustin]

Could try opening the bleed valve and releasing fluid...but this assumes they were bled with the pads worn.

Reset pistons with pads in, remove pads, examine pistons when lightly pulling the lever. Try resetting without the pads in there. See if one piston seems to be stuck, or if both are. If one keeps hanging up, repeat the process to see if it eventually just won't push back in at all. That's how I was able to determine that one of my pistons was cracked on the backside. Similar experience between 3 Ultegra calipers and 1 D/A caliper.

You shouldn't ever get to the point where you're thinking "man...this takes more force than I expected."

[Chik]

Thanks for the replies. I'll have another tinker in the next couple of days.

What mystifies me is the fact that it wasn't a problem with the front caliper. Hopefully, there is no issue with the pistons in the rear caliper -- it's not like spare parts are plentiful these days. The good news is that it's functioning just fine at the moment with the pads that used to be in the front caliper.

[DBordewisch]

If your pads were out that far I would bleed the brake. The fluid will be murky and not pink. While bleeding the pistons should easily slide in. take a q-tip with some brake fluid and wipe around the exposed pistons before pushing them in. Make sure to wipe off the piston once it is in place.

[darkmother]

The rear caliper gets a lot dirtier than the front. I always get sticky pistons on the rear caliper of shimano hydraulic brakes. You could try a drop of mineral oil on the seals and see if that loosens things up at all.

[Chik]

I hope to get to it tomorrow...

I used the plastisol coated end of a Park cone wrench to push mine open.

That's not a bad tip, thanks.

Could try opening the bleed valve and releasing fluid...but this assumes they were bled with the pads worn.

Reset pistons with pads in, remove pads, examine pistons when lightly pulling the lever. Try resetting without the pads in there. See if one piston seems to be stuck, or if both are. If one keeps hanging up, repeat the process to see if it eventually just won't push back in at all. That's how I was able to determine that one of my pistons was cracked on the backside. Similar experience between 3 Ultegra calipers and 1 D/A caliper.

You shouldn't ever get to the point where you're thinking "man...this takes more force than I expected."

I suspected that it shouldn't require tremendous force...

It's possible that the pistons are cracked on the backside, but they seem to be pushing out fine (symmetrically, based on observation with naked eyes). The difficulty in pushing them back in seems the same on both sides -- this isn't based on precise measurement, just by eye.

It was bled once before, but when the pads were much less worn. (The fluid was still clean, but there was a bit of air that needed burping.)

If your pads were out that far I would bleed the brake. The fluid will be murky and not pink. While bleeding the pistons should easily slide in. take a q-tip with some brake fluid and wipe around the exposed pistons before pushing them in. Make sure to wipe off the piston once it is in place.

I suspect you're right about having to bleed. Sorry, what does the fluid do on the piston?

The rear caliper gets a lot dirtier than the front. I always get sticky pistons on the rear caliper of shimano hydraulic brakes. You could try a drop of mineral oil on the seals and see if that loosens things up at all.

Yep, they get filthy, particularly around my neck of the woods during colder months.

Thanks, all!

[Chik]

Use a plastic tire lever without the pads in the caliper, but be careful. If the piston is crooked it can crack. I use pedros ones because I like the colours they come in

The pins fall out all the time. Shimano doesn’t list then as a normal replacement part. If you are worried run the cotter pin you bend. I’ve seen the retention rod fall out, and it is really bad.

When swapping your pads articulate the pistons and make sure they move properly. My procedure is as follows: have the caliper loose on the frame. Put the bleed cup on the lever. Reset the pistons without pads. Put the pads in, and put the wheel in the frame. Pull the lever a bunch to get any air out. Check the caliper. If one piston has moved more than the other, do a caliper service. Line the brake up and off you go.

To bed your pads faster, you can do this: lightly sand them with clean emery cloth (just rough them up). Run them under water, then rub them together so they get kind of gooey. Pat them clean on a clean cloth, and install. They will grab within a few pulls of the lever.

Interesting tip on the bedding in, thanks.

Sorry, what's a cotter pin?

The pads seem to move properly and symmetrically, just doesn't want to go back in fully. It goes back a bit, but not enough. Before I bleed, I'll put on the bleed cup and see if I can push the pistons back just by doing that.

Again, sorry, what is meant by "caliper service"? You're addressing a disc brake Fred so bear with me.

Thanks.

[andrew flowers]

Shimano supplies a threaded pin with a safety clip with new brakes. The safety clip gets lost all the time, but isn’t a stand alone repair part one can order. More a meta gripe of mine unrelated to your issue. The cotter is the two arm pin that comes with pads- just bend the longer one so it can’t fall out.

Caliper service refers to the lubing of pistons. In the shop I facilitate this with modified bleed blocks, etc, seeking to articulate one piston at a time so it can be cleaned and lubed.

If the pistons feel stiff don’t push so hard. Take a photo so we can see how far they stick out right now. Things are probably fine. I generally only replace calipers once things have really got out of hand

[DBordewisch]

Shimano used to suggest a "pro tip" of using brake fluid to lube the piston. With the pads out, wheel off, let the piston extend 2-3mm. then take a q-tip dipped in brake fluid. Swab the exposed part of the piston the retracts into the bore. The idea being this lubricates the piston. Allowing the piston seal to move with less resistance. Then once the caliper is ready for pad installation be sure to clean the area to remove any mineral oil.

[Mark Kelly]

The cotter is the two arm pin that comes with pads- just bend the longer one so it can’t fall out.

Another interesting difference in US usage: in the rest of the world that's a split pin, a cotter pin being one with a small wedge on one end:

s-l640_1_5.jpg


velogear.com.au/9-5-x-42mm-cotter-pin

Commonly used to hold crankarms onto the axle: when I were a lad square taper cranks were known as cotterless cranks.

[andrew flowers]

Confusingly both are called cotter pins

[sk_tle]

We call the one for crankarms clavette in french. Don't know why but I like that word.

[DBordewisch]

Like one of these.

[Chik]

Problem solved, but not in an orderly manner. I was observing the piston movement, got a bit carried away, one piston popped out, and had fluid all over the place. Anyway, I cleaned and lubed the pistons and replenished the fluid in the system. All is well now with the rear caliper.

I thought that I might as well bleed the front as well and found that the fluid was contaminated, so that was good to do a full bleed. Also cleaned and lubed the pistons, but the inner one seems to be slightly more eager than the outer one even after repeatedly resetting and articulating. I don't think it's an issue, but if I need to revisit it, please let me know.

I noticed that the silicon cap for the front caliper's bleed valve is missing. Should I be concerned?

One question on bleeding. A later step in the procedure prescribed by Shimano entails connecting a tube with bag to the caliper whilst squeezing the lever. I get the idea of expelling air sitting around in the caliper end, the pressure in the system seems to decrease significantly as a result, leading to a much longer lever pull for the brake to engage. I suspect I'm doing something wrong. (There is sufficient fluid sitting in the cup at the lever.) Or, is this normal?

[dashDustin]

One question on bleeding. A later step in the procedure prescribed by Shimano entails connecting a tube with bag to the caliper whilst squeezing the lever. I get the idea of expelling air sitting around in the caliper end, the pressure in the system seems to decrease significantly as a result, leading to a much longer lever pull for the brake to engage. I suspect I'm doing something wrong. (There is sufficient fluid sitting in the cup at the lever.) Or, is this normal?

This is really difficult when bleeding an internally-run frame where the hose exits right before the mount. Curious to hear how others do it efficiently.

To answer your question, after expelling the air, and with the bleed port now tightened, squeezing the lever should draw fluid into the line at the MC. It'll take a few squeezes, but it should firm up. If it doesn't, there's air somewhere in the line still.

PS - half of our caps are missing. it's not an issue...just inspect every so often to make sure nothing is lodged in there that might hamper your bleed process.

[andrew flowers]

Yeah worst case pull the whole bleed nipple for your gravity bleed. I do this pretty commonly on mountain bikes

[Chik]

This is really difficult when bleeding an internally-run frame where the hose exits right before the mount. Curious to hear how others do it efficiently.

To answer your question, after expelling the air, and with the bleed port now tightened, squeezing the lever should draw fluid into the line at the MC. It'll take a few squeezes, but it should firm up. If it doesn't, there's air somewhere in the line still.

PS - half of our caps are missing. it's not an issue...just inspect every so often to make sure nothing is lodged in there that might hamper your bleed process.

My frame builder flatly refused to do internal routing, for which I am eternally grateful.

But yeah, gave it plenty of squeezes afterwards but not much improvement. It was nice and very firm before the burping at the caliper end. To put another way, if it is nice and firm, then should I just skip the caliper burping part?

And, sorry, what's MC?

Yeah worst case pull the whole bleed nipple for your gravity bleed.

Sorry, that sounds Finno-Ugric to my ears. What does "pull the whole bleed nipple for your gravity bleed" mean?

[dashDustin]

My frame builder flatly refused to do internal routing, for which I am eternally grateful.

But yeah, gave it plenty of squeezes afterwards but not much improvement. It was nice and very firm before the burping at the caliper end. To put another way, if it is nice and firm, then should I just skip the caliper burping part?

And, sorry, what's MC?

MC - master cylinder

You're again describing what happened to me when I had a splintered piston...when squeezing the lever, the piston would move out, but then would get stuck out, creating need for more fluid, which would cause the lever throw to increase.

But...at this point, I don't think that's a problem. I don't think. When I was setting up the brakes on my bike, I just couldn't do the burping thing. Arms not long enough, frustrated, tired, etc etc. So I pushed fluid up, gravity bled down, then did the whole squeeze-the-lever-a-million-times-to-get-bubbles-in-the-cup. Twice. Not efficient at all, but the bleed has held well.