The system lubricates the pistons from the inside-out. Applying fluid externally can lead to inadvertent pad/rotor contamination or too much piston slip. See DMVancouver's bit below. FWIW, I use a chunky pad spacer (the old Hayes ones work a treat). Leave one pad on the piston, insert pad spacer, and pump the brake lever until the piston without the pad on it contacts the spacer. Remove the spacer and pad, push the piston back in, and repeat the process 2-3 more times, then do the same thing to the other side. This works on 2 or 4 piston brakes regardless of manufacturer, and has taken care of much of the wandering bite point issues I had on one set of my SLX brakes and my GRX ones as well. My other set of SLX's required a bit more work and tinkering as the problem reared back up worse on a night ride in 0F temps.
March 15, 2021, 6:59 a.m. - JT
The system lubricates the pistons from the inside-out. Applying fluid externally can lead to inadvertent pad/rotor contamination or too much piston slip. See DMVancouver's bit below. FWIW, I use a chunky pad spacer (the old Hayes ones work a treat). Leave one pad on the piston, insert pad spacer, and pump the brake lever until the piston without the pad on it contacts the spacer. Remove the spacer and pad, push the piston back in, and repeat the process 2-3 more times, then do the same thing to the other side. This works on 2 or 4 piston brakes regardless of manufacturer, and has taken care of much of the wandering bite point issues I had on one set of my SLX brakes and my GRX ones as well. My other set of SLX's required a bit more work and tinkering as the problem reared back up worse on a night ride in 0F temps.