1999-2004 Honda TRX400EX FourTrax Service Manual, Page 214Get this manual
HYDRAULIC DISC BRAKE CAUTION:
Be careful not to damage the piston sliding surfacePush the dust seal and piston seal in and lift them outRemove the O-ringClean the seal grooves, caliper cylinder and piston with clean brake fluid
INSPECTION
CABLE LOCK BRAKE Remove the shaft boot and the brake shaftCheck the shaft and base threads for wear or damage