I’m with Racer on his suggestions. When ABS was first coming about, I used to ‘force’ piston back into the caliper and leave cap off of master cylinder.
On my first two vehicles with either partial or full ABS, when I did, the ABS light came on. The only fix was to then bleed all four wheels even if I had serviced all or not. After a couple of events like this, I now only return piston with bleeder valve open discharging old fluid through a clear vinyl hose into old plastic pop bottle.
I’ve developed the habit of bleeding regardless when I service brakes and have not had any problems since. Later models with ABS recommend the brake fluid should be changed and system flushed every XXX miles depending on manufacturer.
I admit, I don’t routinely change my brake fluid, but I normally keep a vehicle until I’ve replaced brakes all around so have typically bled/flushed the system at least 2X.
When I bleed the brakes, I use the same clear hose, discharging into pop bottle keeping hose end submerged in old fluid watching for air (valve open on pedal down stroke; closed with pedal coming up.) Once the air bubbles are no longer visible in bleed hose (make sure to check the master cylinder often and keep cap while bleeding).
Granted, the 'old school' method, but works ok on passenger vehicles and light trucks. I'm to cheap to buy an actual brake bleeder since I only need one every few years. Probably could have justified by now with the 'fleet' of family vehicles I've maintained over the years.
Anyway, just my thoughts based on my experiences. Dave