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Disc brake piston position?

6K views 19 replies 5 participants last post by  upstateSC-rider 
#1 ·
New to disc brakes on the road and have always taken the mtn bikes to the LBS for brake maintenance. So I'm learning and want to start maintaining the brakes as well now that I have them on my road bike.

What I'm trying to determine is if the brakes were overbled by the shop who assembled the bike. When the pads removed and the lever not being depressed, are the pistons supposed to be flush with the caliper body or do they normally stick out a mm or so?

Anyway, I'm trying to move to Ultegra rotors with resin (L03A) on the new bike from the XT (RT81-S) that came on the bike with metal (K04S) pads. The XT rotors and metal pads are screeching to no end. Brake calipers are R8070 flat mount.

Anyway, when installing the L03A resin pads and the new Ultegra rotors (which are the same thickness as the XT ones) the pads which are thicker, contact the rotor on both sides. I have not squeezed the brakes with the wheels out. I noticed the pistons stick out about 1 mm from the caliper walls and I can't seem to push them back. I'm afraid I will need to open the bleed port to do this - if they are overbled. I cannot find anywhere stating they should be flush or not and wanted to ask before making a potential mistake.
 
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#4 ·
I can confirm that the pistons should be flush w/ the caliper body. I try pushing them back in w/ a tire lever first. If that doesn't get it done I lube the visible part of the piston w/ mineral oil and then CAREFULLY work them back in w/ the box end of an 11 or 12mm combo wrench. Very carefully.
 
#7 ·
Thanks cxwrench and nova_rider. I will try again with 1st cleaning around the exposed pistons and then trying to press them in flush with a tire lever. If that doesn't work, is it safe to open the bleed port with the funnel and then press them flush?
 
#8 ·
You can't bleed fluid out of a Shimano brake that way. It's a closed system so you'd have to push what you think is extra fluid out of the caliper. You can pump the lever all you want w/ the bleed screw out and you won't lose a drop of fluid.
 
#9 ·
Thanks. Wrapping my head around it the whole system concept. I appreciate your patience.

If I press on the pistons themselves, would that force any excess fluid up to the caliper and out via the open bleed port or would I need to use the lower bleed port on the caliper to pull excess fluid out with the syringe?

Also do you have a recommendation/preference on a hydraulic hose cutting tool and finisher - preferably one that is applicable to Shimano and SRAM?
 
#10 ·
Read my last post again. NO fluid will come out of the bleed port at the lever unless you push it from the caliper. If you want to remove fluid from the system it has to come out at the caliper. You can pull it out w/ the syringe or push it out by pulling on the brake lever OR pushing the pistons back into the bores.
Cutting tools? Lots of good ones, they all use basically a razor blade to cut and have some way to hold the hose. Jagwire, SRAM, or the Shimano BH62 which cuts and also pushes in the barb.
 
#19 ·
Good information from all. Thanks. Nova, I do have a bleed block just haven't done anything bleeding related yet and I do understand it's usage when bleeding. I was just trying to ascertain my starting point and understand the systems better. The bike is new and only has a couple of hundred fair weather miles max on it so pad wear should be minimal. Aclinury is correct in what I was asking. cxwrench, as always, a font of knowledge and experience. I ALWAYS learn something from posting here - I really appreciate it.

I haven't had much chance to mess with the brakes yet in regards to trying to push out the pistons back into the bores. I hope to get to that this evening after work and will report back if I am able to push the pistons back out for the thicker pads.
 
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