Brake pistons creeping out

gingerroad

HI
I have a pair of fairly new 2020 Deore brakes. They haven't had much use, maybe 200 miles or so.
Rear brake is fine, front is a problem.
When set up at home, the rotor spins freely, but after a few proper stops (i.e. first 10 minutes of a ride) the rotor starts dragging on the pads, tends to get worse as a ride goes on so that it's rubbing continuously, eventually quite loudly. Sometimes a few (maybe 10-20) pulls of the lever sorts it out but only for a few minutes. This happens very early on every single ride.
The brakes are new, so I doubt the pistons are corroded or that there's a problem with the seals.
I have pushed the pistons out a little and cleaned/lubricated their sides with some brake fluid. That hasn't helped. They don't seem sticky. Both pistons are moving freely, they just don't seem to return to their starting points.
I have shortened the hose since buying them, using Epic's instructions here: - https://epicbleedsolutions.com/blogs/guides/shorten-shimano-hoses-without-bleeding
The lever feels firm and the throw is about right. the brake is working totally fine apart from this irritating problem.
Anyone got any ideas what might be causing this, and what i can do to fix it please?
Thanks,
Richard.

mully79

Maybe too much fluid in the first place causing the pads to lean on the disc. If they rub enough to heat up the heat expands the fluid and forces the pads harder onto the disc.

reaperactual

I agree with Mully as I had a similar issue once. I didn't recess pistons enough when putting in new pads and had slightly overfilled with fluid doing a lever bleed a few months before.

Remove the lever bleed port screw and wrap a rag around the bleed port.

Fully push back the pistons with something plastic (a tyre lever works well), they should end up near enough flush with the caliper face when done.

Any excess fluid will be expelled. Replace the bleed port screw and put it all back together. A few pumps of the lever and you should notice improved pad, rotor clearance.

gingerroad

Thank you both, i will try that today.

vfr800

Did you shorten the brake hose at all?

vfr800

mully79 said:

Maybe too much fluid in the first place causing the pads to lean on the disc. If they rub enough to heat up the heat expands the fluid and forces the pads harder onto the disc.

Hydraulic brakes are designed to be full of fluid, no air. The fluid is designed not to expand, it's air in the system that will expand the most.