- Car Care & Maintenance
DIY Auto Service; Performing a Disc Brake Job
Hydraulic Brake Three Part Series
Hydraulic Brakes are covered in the following three Hubs.
Floating Disc Brake Caliper
Disc Brake Operation
Disc Brakes use a clamping action on the brake rotor similar to the clamping force used on bicycle hand caliper brakes. Hydraulic piston or pistons provide the clamping forces. The brake pads are located on both sides of the rotor.
The Disc Brake Caliper is used to create the squeezing action on the brake pads. One to four pistons are used on cars and light trucks while typically two pistons are used for medium duty trucks. Since hydraulics are being used, if two pistons each were about 3.5 square inches, the braking pressure would be multiplied by a factor of 7 (3.5 + 3.5).
Truck and Trailer Systems Textbook
Two Piston Floating Caliper
Floating, Sliding and Fixed Caliper
The Floating Caliper rides or floats on slide pins. This allows the caliper to easily slide back and forth. The piston or pistons are located towards the inside. On cars, light and medium duty trucks a one or two piston floating caliper is very popular. As the hydraulic pressure pushes the piston out, the piston pushes the inner pad against the rotor. The piston continues to put pressure on the inner pad and slides the caliper toward the inside until the outer pad applies against the outside of the rotor. Now that the inner and outer pads are forced against the rotor, the rotor has a clamping force applied. The friction material on the pads has a specified coefficient of friction to stop the vehicle. When the hydraulic pressure is released the clamping force is removed. The brake pads are moved away slightly by the movement of the rotor. Since disc brakes do not have any return springs, the brake pads stay very close to the rotor.
The Sliding Caliper action is the same as the Floating Caliper except instead of sliding on pins, the caliper slides on a metal to metal contact. This causes the caliper to be harder to apply and release. This design is also more prone to corrosion causing the metal to metal surfaces at the caliper to freeze up. When removing and installing this caliper, clean the slides and put a light coat of anti-seize on the slide surface. Do not put a lot of anti-seize as it could end up on the rotor. The pistons are also located only on the inside portion of the caliper.
The Fixed Caliper, like the Meritor Four-Piston Quadraulic disc brake caliper has the caliper housing fixed to the torque plate (Bolted Solid). The 4 pistons are located 2 on the inside and 2 outside of the rotor. All the pistons move towards the rotor to form the clamping action.
Disc Brake Inspection
Perform a Visual Inspection, of the rotor, to check for scoring, heat checking (small surface cracks that do not penetrate), cracking and hot spots (bluish/black).
The Rotor Minimum Thickness is typically cast right into the rotor. The minimum thickness of the friction area is critical to the amount of heat the rotor can dissipate. Measure the rotor in three spots around the rotor and the smallest measurement is the minimum thickness. A rotor that is below the minimum thickness should be replaced.
The Rotors should also be matched for thickness on each axle to prevent brake pull due to different heat dissipation. Never just “Slap Pads” on rough, scored or hot spotted rotors. Always service the brakes in axle pairs.
Thickness Variation or Parallelism problem causing a brake pulsation. This is from the rotor being warped thinner and thicker at different points. Since this is connected to the driver’s foot thru the hydraulic system, the driver may also feel the pulsation in their foot. Measure the rotor in three spots around the rotor and a true rotor will have the same measurements. To find the thickness variation, subtract the lowest reading from the highest.
Example: Readings; #1 = 1.452”, #2 = 1.458”, and #3 = 1.453”
Subtract; 1.458 – 1.452 = .006”
This is the thickness variation. Generally .002 to .003” is typically allowed before the rotor needs machining. Excessive Thickness Variation can cause a pulsing in the brake pedal.
Warped causing Run-out or Wobble. With the rotor still on the vehicle, mount a dial indicator so the plunger is on the friction surface. Zero the dial indicator and rotate the rotor. Observe the dial indicator. If the needle moves back and forth over 6 lines, that is .006”. The rotor has a run-out of .006”. Check the manufacturer’s specifications for maximum rotor runout.
Once the wheels are removed and depending on the caliper style, the caliper can usually be removed by removing two to six bolts that hold it to the Torque Plate or Anchor Plate. The caliper can then be slid off the rotor. On sliding calipers, the slide needs to be driven out from between the Anchor Plate and the caliper.
Service Tip: When pushing the caliper piston in to install new pads, it is recommended the bleeder screw be opened to push the old brake fluid out instead of back thru the system.
The flexible brake line is connected to the caliper by a hollow bolt and has a copper or aluminum crush washer on either side of the line.
NOTE: If the caliper is not being removed from the vehicle, use a coat hanger to support the caliper. DO NOT let the caliper hang by the hose.
Hubbed and Floating Rotors
Rotor Removal and Installation
Two types of rotors are most popular; Hubbed and Floating.
Hubbed Rotors have the rotor and wheel bearing hub together. To remove the rotor, the wheel bearing must be taken apart. To reassemble the rotor, the adjustable wheel bearing must be cleaned, packed and adjusted. A new seal should be installed as well. To adjust the bearing typically uses a four step process.
- Tighten the wheel bearing adjustment to specs while rotating the hub.
- Back off the adjustment one turn.
- Retighten the adjustment to a lower torque spec while rotating the hub.
- Back off a specified amount, typically 1/3 to 1/4 turn. Check the bearing endplay, should be .001 to about .003".
Floating rotors are held onto the hub by the wheel. It sits over the wheel studs. Once the wheel is removed, the rotor should slide off of the hub (unless rusted in place). Some manufacturers use clips to hold the rotor in place during manufacturing, cut the clips off. An air chisel with a hammer bit is useful to loosen a frozen rotor. Do not damage the wheel studs.
Once the caliper has been removed, it may be necessary to unbolt the torque plate from the spindle to remove the rotor. Check the manufacturers specifications for reinstalling the torque plate because the torque specification is critical and Locktite (thread lock) is typically used.
Note: When installing the wheels, always torque the wheel lugs to specs in a star pattern to prevent rotor warpage.
Disc Brake Pad Replacement
Once the calipers have been removed, the pads can be replaced. Be observant of locations of clips, springs and inserts on and around the pads.
Replacing brake pads is a common occurrence. Match up the brake pads to locate the wear sensors (if equipped) in the same location and the pad direction. The friction material ALWAYS goes towards the rotor. Pads also come in a variety of costs with the cheaper pads typically having a lower grade friction surface (two letter coefficient of friction). Always replace the pads with the same grade as was originally equipped. Spring clips are generally used to keep the pads from rattling and squeaking. Reinstall the clips in their proper direction and location.
Use a C-clamp to push the caliper pistons into the caliper. With multiple pistons, a block of wood or old brake pad can be used with the C-clamp.
When you are ready to reinstall the caliper, make sure everything is clean and grease free. Grease contamination on the pads or rotor can affect the brake performance. Brake cleaner spray works wonders to evaporate the grease and handprints.
Some brake systems install the brake pads into the torque plate while others install the pads in the caliper. Make sure the spring clips are installed properly or the pads may squeak and rattle. Once the pads are installed, install the caliper onto the torque plate. Lubricate the pins or slides with the appropriate grease and torque the slider bolts to specs.
When all of the calipers are installed, pump the brake pedal to bring the pistons out and push the pads against the rotor. Failure to do this could cause no brakes when the vehicle is put in gear until the brake pedal is "pumped up".