If rotors are true, free of...
If rotors are true, free of scoring, and measure the correct thickness, a pad change can be made without disturbing the wheel hubs. This is also a good time for a bearing clean, inspect, and re-pack. Either way, the job begins with caliper removal.
A shop manual will describe the steps for removing your truck's calipers. Among service tools, a caliper clamp will press the piston back into its bore. This is necessary to establish room for the new, thicker pad lining. The caliper can't slide over the rotor without adequate clearance between the new pad linings and the rotor faces.
If you suspect a warped rotor, check the runout with a dial indicator. A good measure is to adjust the wheel bearings for zero endplay before testing runout. Use a magnetic stand or clamp to hold the dial indicator. Make sure the dial indicator is firmly mounted, and use a roller tip to run against the rotor face. The indicator stem should be perpendicular to the rotor, or tilt it slightly to prevent chatter.
This caliper piston was frozen....
This caliper piston was frozen. Prying gently at the pad pushes the piston inward and provides space for removing the caliper. Pushing the piston into the caliper bore forces fluid back to the master cylinder. If necessary, relieve excess fluid at the bleeder. Re-tighten the bleeder to prevent contaminants from entering the system.
Turn the rotor slowly, and observe the lateral runout. A typical factory runout limit would be 0.004 inch or less. Equally important is the "parallelism" of the rotor faces. This is the thickness of the rotor, measured with a quality micrometer. Thickness should be uniform within 0.0005 inch (half a thousandth of an inch). If the rotor needs surfacing, remove the hub/rotor assembly (rotor only on some applications), and sublet this chore to a machine shop.
If wear is within specification, readjust the wheel bearings. If bearings need service, remove the hubs and perform a bearing pack. (See last month's lesson for wheel bearing details.) Clean and dry the hubs and bearings thoroughly. Re-pack with high-quality wheel bearing grease, rated for the high-heat output of disc brakes. Wheel bearing endplay adjustment is critical with disc brakes. Rotors must run true. Wheel bearing adjustment effects runout. A dial indicator is the tool for setting endplay.
If only the pads need changing, air should not get into the hydraulic system. A vacuum bleed and flush with fresh DOT 3 or 4 disc brake-approved fluid is advisable. (Use the fluid recommended for your hydraulic system.) Brake fluid draws moisture at approximately 3 percent per annum, and this lowers its boiling point. Fresh brake fluid, pulled through the system with a vacuum bleeder, will restore the boiling point, remove contaminants, and help prevent brake fade.
Never suspend a caliper by...
Never suspend a caliper by its hose. Tie the caliper out of the way with mechanic's wire or welding rod. If only the pads need replacing, this is as far as the caliper needs to move. Safely clean brake dust away from the caliper before and after removal.
When calipers seep fluid or the caliper seal appears weak, there is a need to rebuild the caliper. Disconnect the hose at the caliper and head for the workbench. To remove a stubborn piston, one procedure is to use compressed air at the hose inlet. Make sure the piston points away, and use short bursts of air to coax out the piston. The piston may dislodge with force, so be cautious! Let the piston land in soft rags. Keep your hands out of the way.
Use denatured or isopropyl alcohol to clean the parts. Vehicle manufacturers do not advise honing a caliper. Instead, they recommend the use of crocus cloth to hand-smooth slight bore blemishes.
Most pistons have a thin nickel/chrome coating. Never attempt to wire brush orsand the piston surface. If rusted or scored, replace the piston or caliper unit. When prying out the seal, do not damage the bore. Measure the piston-to-bore clearance and compare it to factory limits. When wear is excessive, there are new or rebuilt/exchange calipers available
Clean up the caliper bore with denatured alcohol and coat with brake fluid prior to assembly. Use care when assembling the caliper. Some calipers have two-piece shells. Replace seals or O-rings between the mating surfaces as part of the rebuild. Use a factory or professional manual when performing disc brake work. Although less complicated, disc brakes rely upon close tolerances. A factory-level shop manual will provide accurate service details.
 Compressed air can be used...  Compressed air can be used to remove the caliper piston. Keep hands out of the way. Point the piston in the opposite direction. A block of wood (shown) or soft rags can catch the piston. Apply air cautiously through the hose opening. |  Pry the seal away carefully....  Pry the seal away carefully. Do not scratch the caliper body or the bore. A rebuild kit will contain the new seal and a piston seal. Handle these parts carefully. Clean parts with denatured or isopropyl alcohol. |  Examine the piston for rust,...  Examine the piston for rust, corrosion, or scoring. Check the bore for pitting and scratches. Pressure is high in the caliper. Any warp or distortion can cause seizure of the piston. |