Part of Lesson Plan: **Brake Measurements and Hydraulics (updated) by Curt Erales
Activity Overview / Details
When drum or disc lining is replaced, it is common practice to resurface the drums or rotors using a drum or rotor lathe. If too much metal needs to be removed to clean up its surface, the drum or rotor needs to be replaced.
Most rotors and drums have casting or stamping listing the minimum thickness. There is brake rotor and drum book of specifcations where a student can look up the minimum specification that a rotor and drum can be machined before it can be discarded. These specifications can be in inches or milimeters. There is a handout with a conversion chart with the brake specifications from inches to millimeter. One inch equals 25.4 mm when using a conversion scale.
When using a brake rotor caliper, the caliper is placed over the rotor and adjusted all the way in to its lowest point on the rotor even if there are grooves in the rotor. This is where the most accurate reading is taken.
A drum is measured with a special caliper with a dial indicator. The maximum amount that can be cut from most drums is 0.060 inches. The drum micrometer is first adjusted to the size of the drum. It is then put inside the drum and moved side to side, and the highest reading is the valid reading.
The thickness of a rotor must be measured to see if it is still usable. There are special micrometers available that have a pointed tip so the depth of the groove can be measured. This will save machining a rotor that is too thin, which is 0.015" above the manufactor specification.
The brake rotor is also checked for excessive runout with a dial indicator. The maximum runout specified is 0.005" even though some manufactors specify less. One way to use the dial indicator is to attach it to the spindle and allow the dial indicator to touch the rotor. Spin the rotor slowly. The maximum runout should be a miximum of 0.005"; if it is more, the rotor has to be discarded.
Look for physical damage to hoses, such as cracking, swelling or softening from exposure to oil. Sometimes a hose that was installed improperly can rub until it fails. Also, a partially plugged brake hose can cause the car to pull at the beginning of a stop; when the presure equalizes, the pull goes away.
The hydraulic system should also be checked for leaks. When inspecting a caliper there should be no signs of leakage at the caliper piston seals. It is important to check the dust boot to see if it is torn or leaking. Look for physical damage to hoses such as cracking, swelling or softening due to exposure of oil.
When the master cylinder fluid level is low, look for an external leak.
Check the wheel cylinder for leaks, pull back the dust boot and check for leakage. A small amount of dampness from brake fluid is acceptable but fluid should not be dripping.
Materials / Resource
- chkrotor.gif [ View Image ] [ Download Original ] Checking the measurements on a brake rotor.
- BrakeLines65.jpg [ View Image ] [ Download Original ] Diagram showing the brake hydraulic system.
- brake_system.jpg [ View Image ] [ Download Original ] Handout showing Brake lines.
- checking brake rotor runout.jpg [ View Image ] [ Download Original ] Checking the Brake rotor runout.