1986-1989 Honda TRX250 FourTrax 250R Service Manual, Page 186Get this manual
HYDRAULIC DISC BRAKES Position the caliper with the pistons down and apply small squirts of air pressure to the fluid inlet to remove the pistons
Do not use high pressure air or bring the nozzle too close to the inlet Place shop towel over the piston to prevent the piston from becoming nyo;ectile
Push the dust and piston seals in and liftthem outClean the seal grooves with clean brake fluid
CAUTION
Be careful not to dama)!e the piston slid;n)! surfaces
INSPECTION
Check the caliper cylinders and pistons for scratches, scoring or other damageMeasure the cylinder inside diameter and piston outside diameter