You can fix stuck or sticky caliper pistons by using the SRAM pad advancement procedure.
Over time, dirt and grime can keep pistons from moving freely. This can result in pads not advancing far enough to provide adequate brake power, or pistons not retracting.
You can find the procedure in our SRAM brake manual.
TECH TIP: Do not lubricate the pistons. Lubricating pistons reduces the friction between pistons and the square seals that pull them back into the caliper for consistent pad retraction.