Why is my rear brake binding

My rear brake binds likek a sod, if i (completely) respace the pistons it’s ok until i use the brake then it doesn’t pull back off the rotor fully.

It was formula r1 brakes with matching rotors, i swapped them for a pair of Magura mt6 which did the same (maybe a little better). Back on the formula following an expensive interaction with the ground and they’re binding something horrible again.

I’ve stripped and rebuilt the calipers, including greasing the pistons seals etc. They visibly retract but won’t pull back off the rotor far enough.

Sliding dropout frame with is mounts.

Same issue with the qr mounts when ss’d as with the through axle mounts geared so I’m comfortable the issue isn’t the mounts themselves.

I’m tempted to give the is adapter a tickle with a bench grinder and see if it’s a bit thick so pushing the caliper too far over.

Any ideas would be welcomed, especially by my legs.

Are both pistons binding, or just one side?

How are you centering the caliper over the disc? Pulling the lever and letting the caliper centre itself, or doing it so that the disc sets in the centre of the caliper?

Is the caliper centred on the disc and is the binding always on the same side?

If the caliper is already correctly centered then there’s nothing to be gained by attacking the adaptor (IS to Post, I take it?). If one pad always fails to retract completely then it must be a seal/piston issue, I’d have thought?

My last bike refused to let me centre discs by pulling the lever, I had to centre them by eye. Could be that? Could also be a bent rotor that’s pulling to one side? The fact you are having the issue with two different brake sets suggests its either rotors or alignment.

 How are you centering the caliper over the disc?

I’ve tried both centering by eye and also letting the caliper do it’s own, both have the same result.

The guess it’s just one side binding though I’ll be honest I’m damned if i know which for sure, both visibly retract.

I’ll rebleed this weekend any how as i clearly over filled when i rebuilt it last time (rotor was steaming whilst pedaling last night, is never been that bad before) but think I’ll attack the mount too if there’s no lightbulb moment suggestions.

I can’t really understand the “they both visibly retract and yet they still bind” bit, unless the rotor is distorted or bent. However, you’ve tried two different brake set-ups and had this happen with both.

Does the adaptor locate the caliper exactly square to the disc, both vertically and fore and aft?

What did you grease the caliper seals with, btw?

Red rubber grease for the seals (last night i was sorely tempted to try a bottle of wet lube.)

Caliper looks to be running in plane with the rotor, as mentioned though, two sets of dropouts giving the same issue so i can’t see it being “misaligned” on both (three sets of wheels too).

Visibly retract maybe smoke and mirrors on my part, once re assembled after strip and rebuild I actuated them in hand a few times (with a bleed block in place) the pistons can be seen to move out then disengage and retract back from the block – only small mind, I’m guessing travel on the pistons between off/dental damage is maybe 0.5mm either side. I can’t see them retract on the bike as the pads, rotor frame etc make that 1mm near invisible, i might manage it by sticking a bright light under it but think I’d not be seeing much else for a few hours.

I had this recently on a Shimano SLX – I got all obsessed with it being the caliper then eventually it dawned on me that it was actually the lever that was sticking. A couple of drops of oil and the problem has gone.

I’m only posting this because you have probably already ruled this out, but if you haven’t, and it is such an easy fix then you might thank me!

Formula brakes IMO retract a lot less than some others. I was doing my mates the other day and noticed he had a fraction of a mm on both sides clearance. I rekon on my Shimanos i have 1mm each side.

Its a good one to check certainly. Less likely to result in loss of fingers than a bench grinder certainly…

Edit I had considered that weeksy, i might try a set of shimano, if nothing else in due define shiny stuff.

When you say stripped and rebuilt – did you replace pistons and seals or just lube?  If the latter then I would replace with new.  Cleaning and lubing may work for a period of time but if they’re goosed it won’t last…