For the hydraulic booster, which enhances braking power, on certain vehicles, air tightness between the piston and the piston cup in the cylinder is poor due to the improper processing of the piston cup groove. For this reason, air may be sucked through between the piston cup and the piston because of the vacuum created by repetitive operations of the brake pedal, and it could mix into the brake fluid. At worst, damping force decreases. Note: This problem was reported in RJ-1168-0, submitted on 21st July 2004, but the vehicles to be recalled were not fully accounted for, so this recall is to add some vehicles to the recall list.