1: Kawasaki sells the individual parts to rebuild your master cylinders and calipers, so let?s completely rebuild the KX125 front brake system. Rear brakes differ from the front in that the M/C has a removeable reservoir and the caliper is usually a single piston. Loosen the brake-line banjo bolts, then remove the M/C, line and caliper from the bike.
2: On the master cylinder, remove the brake lever to reveal the M/C seal cover. Pry it out carefully then work the E-clip piston stopper from its groove. Now remove the O-ring stopper and the piston assembly. Clean its housing and look for corrosion in the bore (hone if present).
3: Once the M/C body is thoroughly clean, coat the new M/C piston ($25) with assembly (or brake) fluid and insert into the M/C body. Now reinstall stopper, retainer clip and cover. Reinstall brake lever, fill reservoir with fluid and bench bleed to purge air from around the piston. Reinstall M/C on handlebar.
4: Using compressed air, rags and safety glasses, pop pucks from the caliper as shown. Pry fluid seals and wipers (AKA packing) from body, clean thoroughly and inspect caliper. If puck cavities can?t be honed smooth, the caliper is shot.
5: New front seals and wipers cost about $12, a fraction of the cost of a whole caliper. Coat new rubber bits with fluid and install in appropriate grooves. Partially fill caliper with brake fluid, then press pucks into their seals. With banjo hole facing up, fill caliper with fluid and tap to dislodge air bubbles.
6: Reinstall brake line to caliper, inject new fluid through caliper bleed screw with syringe and tap entire length to dislodge air bubbles. Install caliper on fork and route line to M/C. Refill line before fitting to M/C?back-bleeding (pressing caliper into disc) is the hot setup. Now tighten top banjo and pump the brake lever, being careful to keep reservoir filled, to finish the job. Bleed until mushiness disappears.