Mechanics: How to Improve Hydraulic Disc Pad Clearance?

Triathlon Forum
1

Ok, so I’m heading away on Friday for a weekend of racing, followed by a mini-cycling holiday and I currently don’t have a functional bike so please, I need your help!

To cut a long story short, I have a road bike with hydraulic discs (Shimano ST-RS685 levers and BR-RS785 calipers) and due to an unfortunate series of events, ended up breaking one of the calipers; cracked one of the ceramic pistons, hydraulic fluid leaking everywhere, piston jammed in at an angle, total cluster****.

Fortunately I have a mountain bike that uses very similar calipers (Shimano Deore BR-M595), post mount, same piston diameter, same direct hose connection. I thought, “Fantastic, Shimano are good at cross compatibility, let’s switch one of those in”. Switched them over, bled the system with fresh mineral oil, nice solid lever feel, all looking good.

However, after refitting the pads and wheel, I’m having major issues trying to align the pads and rotor. I must have spent nearly an hour last night trying every method in the book for aligning calipers and whilst I can just about get the wheel to spin freely, as soon as I go through a couple of hard braking cycles, the rotor rub is back. I’ve torqued the caliper mounting bolts tighter than they should be, so I don’t think that’s shifting around, it’s just an issue with the pad clearance being *right *on the limit. After I’ve reset the pistons I’ve been pumping the brake lever to advance the pads with just the rotor in between them, but I notice some people advocate sticking a business card either side of the rotor to add some extra clearance. Does this work, and once you’ve removed the business cards, don’t the pads just advance that little bit further as soon as you brake hard?

One thing that is immediately obvious is that the brake lever has a lot less travel than it did previously, before it was coming maybe 75% of the way back to the bars and now it’s more like 50%. Can I just open up the bleed port on the lever and let out a drop or two of hydraulic fluid to trade a bit of lever travel for some extra pad clearance, or does it not work like that?

Any help gratefully appreciated because I’m now starting to stress out as I count down the hours until I have to leave for the weekend!

2

Just to make sure I am following you, you hooked up the deore caliper to the STI levers right? It is likely they run different volumes for the pistons so the lever may not have enough volume to allow the pads to retract how they would with the mtb levers. The rotors should be the same thickness, but you can throw your mtb one on to confirm that the issue is not rotor thickness and is indeed a volume issue.

3

When you’re advancing the pistons, are you making sure they advance equally? You can push one back in a bit, or hold it still, while the others advance. You want both pistons at the same point when you put the pads back in and then center the caliper.

You can use business cards, or the Hayes caliper alignment tool, to help align he caliper to the rotor, once the pads are installed.

4

Can I just open up the bleed port on the lever and let out a drop or two of hydraulic fluid to trade a bit of lever travel for some extra pad clearance, or does it not work like that?

Yes providing your lever throw is short.

Those R785 levers have a notoriously long throw, even when well bled so if you have a shortish lever throw then you can just crack open the bleed nipple and let a bit of fluid out.

Also, the proper (abbreviated) bleed procedure for those levers is the usual push the fluid up from the caliper to a funnel on the lever. Lock off the bleed nipple. Then do a gravity bleed from the top down with a catch bag at the caliper end, topping up the funnel at the lever with fluid as needed. Lock off the bleed nipple and leave the funnel on the lever end. Flick lever a few times to dislodge any air in the lever.

5

Thanks for the reply.

Just to make sure I am following you, you hooked up the deore caliper to the STI levers right?
Correct.

The rotors should be the same thickness, but you can throw your mtb one on to confirm that the issue is not rotor thickness and is indeed a volume issue.
Unfortunately my mountain bike rotors are 6-bolt and by road hubs are centre-lock so I can’t do that easily, but surely any discrepencies would be discounted once you reset the pistons and advance them again?

It is likely they run different volumes for the pistons so the lever may not have enough volume to allow the pads to retract how they would with the mtb levers.
Indeed, that’s a possibility that I had considered (or feared!), but I’ve come across reports of people having used similar Shimano mountain bike calipers (namely XT and new Deore) with these shifters, so I was fairly confident it would be compatible.

I’m not an engineer, but my logic and limited understanding would suggest that *if *the pistons are the same diameter between the two calipers (I’m reasonably sure that the broken BR-RS768 and the new Deore BR-M595 are both 22mm diameter dual pistons) then pushing the same amount of fluid (i.e. the amount that the road STI levers push) would produce the same amount of pad travel, right? So for example, let’s say the STI lever is pushing 1 cc of mineral oil, you’ll get a total of 1 cc of displacement at the pistons, which for 22 mm pistons would be ~1.3mm per pad. The numbers aren’t important, but so long as the piston diameter is the same between the calipers, the pad travel should be the same, right?

As you can probably tell, I’m still hoping the overly tight lever throw is the source of the issue at the moment and not a fundamental lack of compatibility!

6

Hi, thanks for the reply. The business card trick sounds interesting, but is the pad-spacing maintained once the business cards are removed? Won’t the pads just advance that little bit further when you start using the brakes without that extra spacing in there?

7

is the pad-spacing maintained once the business cards are removed? For the most part, yes. And that’s if you start with equal piston advancement on each side.

8

Yeah, I certainly noticed the long lever throw when I first rode with them; I had a Google at the time and it seemed like the long lever throw was a common complaint, but maybe they’re set up like that for a good reason? As I mentioned, comparing the newly installed front brake with the rear (which I haven’t touched) it’s certainly a shorter lever-throw than it was before.

Thanks for the bleed procedure, when I switched the calipers I used the “inject-from-the-caliper-to-the-funnel” method that you mentioned, but perhaps I’ll try adding on the reverse procedure too; do you mind if I ask what the purpose of this part is, is it just a more thorough process to eliminate any trapped bubbles?

9

comparing the newly installed front brake with the rear (which I haven’t touched) it’s certainly a shorter lever-throw than it was before.And it’s not related to piston position? Advancing the pistons is how you make them bite earlier in the lever throw.

10

is the pad-spacing maintained once the business cards are removed? For the most part, yes. And that’s if you start with equal piston advancement on each side.

Ok, that’s good to know. A lot of people seem to recommend that procedure, so I guess it must have some basis! I’m just a little bit wary that it’ll be fine for the first 10 brake cycles of my race and then suddenly they’ll be locked on again!

11

comparing the newly installed front brake with the rear (which I haven’t touched) it’s certainly a shorter lever-throw than it was before.And it’s not related to piston position? Advancing the pistons is how you make them bite earlier in the lever throw.

Well, once I’d installed and bled the new Deore caliper, I pushed both the pistons all the way back in, reinstalled the pads and wheel and just pumped the lever until the pads started biting; this took maybe 3 full lever pulls. Is this likely to have over-advanced the pistons? As far as I can tell, they were advancing equally (i.e. one of them isn’t stuck).

12

That is a good/correct procedure, especially when the pads are new (thicker).

Remove the pads, advance the pistons a bit, trying to keep them equally advanced. Put the pads back and and feel the lever contact-point change.

13

You will have been given a bleed block with your bike.
This is a yellow block that replaces your pads and rotor whilst you bleed them.
By using this you will ensure that you have not overfilled the system as it is sized correctly for new pads and rotor.
This is to ensure that when you pop new pads in that they go back far enough to fit and that the system has enough fluid to take up the space of worn out pads.

Shimano MTB and road are totally cross compatible.
You cannot change the pad clearence as this is set by the properties of the rectangular seal in the calipers.
Cards and other suggestions only work until you pull the lever a couple of times.
You can overfill them to have less clearance but this will soon normalise as pads wear and you won’t be able to fit new pads without letting fluid out.
The only things you can change are the two adjustment screws in the lever body.

If you are having trouble setting the caliper then I suspect that you have a bent rotor, piston seals are sticking or you have overtightened the caliper bolts and there is now a distortion that makes the caliper want to revert to whatever position it was in when it was overtightened.

Check that the frame mounting surface is indeed flat and that there are not puckering around the mounting holes.
Check that the caliper has not got indents from overtightening.
Make sure you have used the self aligning washers.
Check for free and even movement of both pistons.
Bleed brake with bleed block in.
Fit pads and loose mount the caliper to frame.
Squeeze lever and watch that both pads advance the same.
Hold lever and carefully tighten mounting bolts without moving the rotor.
Spin the wheel and apply the brakes a few times.
Adjust lever feel at the lever for personal preference.

Don’t listen to anybody that says that you can change the way pad retraction operates apart from using different fluid. They are talking out of their arse and don’t understand how pad retraction works.

14

This may be of help to you: https://www.pinkbike.com/news/tech-spotlight-brake-caliper-piston-service-assessment-2014.html
.

15

You will have been given a bleed block with your bike.
This is a yellow block that replaces your pads and rotor whilst you bleed them.
By using this you will ensure that you have not overfilled the system as it is sized correctly for new pads and rotor.
This is to ensure that when you pop new pads in that they go back far enough to fit and that the system has enough fluid to take up the space of worn out pads.

Shimano MTB and road are totally cross compatible.
You cannot change the pad clearence as this is set by the properties of the rectangular seal in the calipers.
Cards and other suggestions only work until you pull the lever a couple of times.
You can overfill them to have less clearance but this will soon normalise as pads wear and you won’t be able to fit new pads without letting fluid out.
The only things you can change are the two adjustment screws in the lever body.

If you are having trouble setting the caliper then I suspect that you have a bent rotor, piston seals are sticking or you have overtightened the caliper bolts and there is now a distortion that makes the caliper want to revert to whatever position it was in when it was overtightened.

Check that the frame mounting surface is indeed flat and that there are not puckering around the mounting holes.
Check that the caliper has not got indents from overtightening.
Make sure you have used the self aligning washers.
Check for free and even movement of both pistons.
Bleed brake with bleed block in.
Fit pads and loose mount the caliper to frame.
Squeeze lever and watch that both pads advance the same.
Hold lever and carefully tighten mounting bolts without moving the rotor.
Spin the wheel and apply the brakes a few times.
Adjust lever feel at the lever for personal preference.

Don’t listen to anybody that says that you can change the way pad retraction operates apart from using different fluid. They are talking out of their arse and don’t understand how pad retraction works.

1+. This is the best answer so far.

16

comparing the newly installed front brake with the rear (which I haven’t touched) it’s certainly a shorter lever-throw than it was before.And it’s not related to piston position? Advancing the pistons is how you make them bite earlier in the lever throw.

Well, once I’d installed and bled the new Deore caliper, I pushed both the pistons all the way back in, reinstalled the pads and wheel and just pumped the lever until the pads started biting; this took maybe 3 full lever pulls. Is this likely to have over-advanced the pistons? As far as I can tell, they were advancing equally (i.e. one of them isn’t stuck).

What was stopping the pistons when you bled the system?
There should have been no need to push the pistons in after. (ie the yellow bleed block basically holds them flush).

It sounds like the system is overfilled.

17

Thanks for all the replies.

I think I’ve managed to solve this now, the problem (as rijndael suggested) was a dominant piston and uneven piston advancement; it seems that after pushing back the pistons, refitting the pads/wheel and then pumping the brake lever, one of pistons was moving much more readily and when I pumped the brake lever basically only this piston was advancing and it was completely screwing up the centering of the pistons and their retraction after braking.

Once I’d made sure that both pistons were advancing a little bit out of their “fully pushed home” positions, they moved a lot more equally, retracting properly and now I’ve managed to centre the caliper without any rub. I’ve taken it out for a quick spin with some hard braking and it seems to be working great; going to take it for a longer ride tomorrow morning, but I don’t want to start fiddling with it again so close to race day!

I’ve learnt a lot about the complexities of hydraulic brakes from you guys and from reading a lot of articles; if anyone ever says that disc brakes are maintenance free please feel free to point them towards this thread and I’ll be glad to tell them how misguided they are!

18

You cannot over advance pads if there is a disc in between them.
Are the adjustments in the lever the same for both front and rear?