Shimano XT or XTR rear brake pump problem

[AntonioGG]

I'm hoping someone here can help me out to tell me if I should just change my brakes.

A little background:  I've been doing brake jobs on my cars for 30 years.  That includes a few years of racing and regular brake bleeds.  I've also used and serviced Hope Race X2 brakes with no issues whatsoever.  I've had older XT brakes that I never had to touch as well.

But these XTR brakes keep giving me a problem.  Online searches result in one of two things:  defective brakes (recall by Shimano) or bubbles.  I really don't think my problem is bubbles because a good bleed doesn't fix the problem.  I have removed the caliper and let it hang under the lever for maximal pressure differential.  I've pumped up with a syringe, and down in the traditional way.  With the block in place, they feel solid. 

Here's the tricky bit:

The thinner my disk or pads get, the worse this problem is.  When I switch to one wheel with my thinnest disk, and with maybe 50% worn pads, I get zero brake no matter how many times I pump the lever.

If I put some new pads in, the brakes feel solid.  It seems to have to do with maybe longer travel being required in the master cylinder.  Pumping the lever results in the pads going in and going back out.

I'm thinking of just grabbing the Hope X2's from my other bike for my race next weekend, but it is bugging me that I can't get these brakes to work well with anything but 75% to 100% new pads.

[crazyt]

doubt this is it, but here is one thing to try related to the freestroke adjustment. Unscrew the freestroke screw, then bleed the brakes, then screw the freestroke in (I guess it displaces fluid to increase the pressure a tiny bit).

 

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Even with a perfect bleed (which I'm sure you did considering your own admission of OCDness) the rear lever is typically going to bite a little closer to the handlebar due to the longer cable. Without going into the physics of it, the longer cable results in more 'throw', as you put it. This is likely related to volume, compressibility and pressure in the (supposedly) closed system.

My solution to get the bite point in the same position relative to the handlebar was to bleed the rear again, but this time do it with the screw all the way (or close to) out. This effectively increases the volume of the cylinder. Do your perfect bleed again (well, just a top up really to get rid of any bubbles) and close the system. Now when you wind the screw down you will find that the bite point moves away from the bar. Not much, but it's noticeable. I view it as effectively increasing the pressure in the system when you wind the screw down.

If you run out of screw on the rear cylinder, match the bite point on the front lever by screwing out. That should get you somewhere close to being matched, if not perfectly. It worked for me.

When you say you pump up and down do you mean  you pushing fluid up from the caliper syringe up to the lever into the screw in cup?

Are you pulling and pushing the plunger from the caliper to get air out of both sides?

 

[Anita Handle]

Did you eliminate the possibility that they are experiencing the issue that Shimano recalled brakes over? Do the recall symptoms match your symptoms?

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[AntonioGG]

Thanks guys for the replies.  Though I don't believe I've resolved my problem, I have additional findings:

1. My disk was down to 1.43mm.  New is 1.8mm and min spec is 1.5mm.  I replaced it, but that by itself didn't fix it.
2. My pads were only worn down 0.5mm each.  That's down to 3.5mm (backing plate included) from 4mm new.
3. One of the YouTube videos showed a master cylinder bleed wherein he flicks the lever to get bubbles out.  Other videos showed them rotating the master cylinder to different angles to get the bubbles out.  I tried the same, and lo-and-behold, tons more bubbles came out.  I have the new disk and new pads installed and I haven't gone back to the 3.5mm pads yet to see if they'll work OK, but I will at some point.

@crazyt, yep, that's what I meant.  The Shimano instructions, as well as several YouTube videos show step 1 is pushing fluid up to the master cylinder (with funnel installed) from the caliper, then letting it drain back down.  Regarding the lever throw, that's another good possibility for me to try.  Maybe my shortened throw makes bleeding harder.  I'll experiment a bit more with this later.

@Anita Handle , as to the recall, my search for this showed it's different brake models that were recalled.  I'll still call Shimano next week (after my race) to see if they have more info or any recalls not published.  The forums out there did seem to imply XT and XTR brakes were affected.

I did some math to calculate how much master cylinder travel goes to displace the volume from pad and disk wear.  About 6.6mm before replacing my parts was just to make up for the wear.  That's pretty significant, especially with shortened throw setup in my lever.

 

[Anita Handle]

man, you are science-ing the hell out of those brakes!

[AntonioGG]

2 hours ago, Anita Handle said:

man, you are science-ing the hell out of those brakes!

I tend to do that if I don't find satisfactory answers to "why?"