Owned a set of the 7000s and dear lord what a PITA. I had a lever develop the slow return you speak of. Once it started to become a major issue I decided to pull both brakes apart. Turned out the slow returning brake's mc piston had swollen up some when compared to the other's, just .2mm in diff between the two. This allowed it to start locking up in the mc bore. It should be noted they are plastic. Brakes were out of warranty, so decided to wing it and ordered a set of aftermarket Ti mc pistons for em. They worked-ish. The dimensions were considerably undersized on one which allowed the piston to rock a bit in the bore under use, causing a seal blowby event. So don't get those aftermarket jobbers. Cannibalized an older set of non-series Deore brakes to find they had aluminum mc pistons that were the same dimensions as the good plastic ones in the SLX brakes. Rebuilt the mc with em and they feel divine, like what the brake is supposed to feel like and better than new ones. They're now on a buddy's bike and working aces still. (of note, I only ever used Shimano's mineral oil in em, so the pistons didn't swell from fluid contamination)
This has reinforced my belief that plastic has no business being used for mc pistons.
Currently waiting on bar stock to turn down a few prototype Al pistons for Shimano's SLX and above units. Apparently current Shimano brakes share the internals from these older generations, so likely only a bit of time before they start suffering the same succotash.
April 19, 2022, 10:07 a.m. - JT
Owned a set of the 7000s and dear lord what a PITA. I had a lever develop the slow return you speak of. Once it started to become a major issue I decided to pull both brakes apart. Turned out the slow returning brake's mc piston had swollen up some when compared to the other's, just .2mm in diff between the two. This allowed it to start locking up in the mc bore. It should be noted they are plastic. Brakes were out of warranty, so decided to wing it and ordered a set of aftermarket Ti mc pistons for em. They worked-ish. The dimensions were considerably undersized on one which allowed the piston to rock a bit in the bore under use, causing a seal blowby event. So don't get those aftermarket jobbers. Cannibalized an older set of non-series Deore brakes to find they had aluminum mc pistons that were the same dimensions as the good plastic ones in the SLX brakes. Rebuilt the mc with em and they feel divine, like what the brake is supposed to feel like and better than new ones. They're now on a buddy's bike and working aces still. (of note, I only ever used Shimano's mineral oil in em, so the pistons didn't swell from fluid contamination) This has reinforced my belief that plastic has no business being used for mc pistons. Currently waiting on bar stock to turn down a few prototype Al pistons for Shimano's SLX and above units. Apparently current Shimano brakes share the internals from these older generations, so likely only a bit of time before they start suffering the same succotash.