It is normal for disk brake pads to be in contact with the rotors - there is no spring to pull them back when you release the brake pedal. The pads push back against the calipers until they're basically no longer compressed at all and will still be in slight contact with the rotors. If a problem exists that keeps the pads in firmer contact with the rotors (e.g. improperly adjusted pedal to master cyl clearances so fluid can't be pushed back into the reservoirs, rust/gunk adding excessive friction to the calipers) then the pads will wear on the rotors. This makes excessive heat which can, depending on severity:
1: lead to an overheated rotor and/or warping the rotor. If you see blue streaks on the rotor it's garbage - it's been really overheated.
2: pads will wear rapidly of course.
3: overheated pads can glaze - getting a hard surface. They'll start making squeak noises when used. Also, high temps in brake pads (especially simple organic pads as well as cheap pads) will boil off some of the adhesive which forms a thin vapor barrier between the pad & rotor... this acts like oil... making the brakes ineffective. This is the most common form of brake fade on cars. (edit: drilled or slotted rotors, like you see on performance cars, help this situation: the holes/slots give the vapor some place to go so the pads can still do their jobs)
4: really hot calipers will eventually cook the brake fluid, causing it to boil. New/fresh fluid has a fairly high boiling temp but, as brake fluid ages (and absorbs humidity which it LOVES to do) the boiling temp reduces. Once the fluid boils it won't transfer pressure from the pedal/master cylinder to the calipers so the brakes are useless again.
Simple tests for dragging brakes:
1: jack the car up, put it on jack stands, parking brake OFF, transmission in neutral. Spin each
wheel. Compare a
wheel to its twin on the other side of the vehicle: how easy is it to spin, how long will it coast. On rear drive Genesis cars, the rear wheels will be a little harder to turn than the fronts. For most other cars (front drivers) the front wheels will be tougher to turn. Any
wheel that spins harder, or won't coast much, compared to its twin on the other side has brake issues. Any
wheel that makes different noises has issues too.
2: easier test: go for a drive on an empty road at moderate speeds. Drive for a few miles. COAST to a stop, put the transmission in Park. Basically, the idea is to NOT use the brakes for 5 to 10 minutes. Hop out of the car and put your hand NEAR TO BUT NOT TOUCHING the
rims (or use one of those infrared heat sensing guns) - feel the heat being radiated from the brake area. Compare a
wheel to its twin on the other side again. Heat = brakes dragging. If you have to use the brakes a few times before you can get to that empty road you'll have to either a) drive on that empty road a long time to cool the brakes or b) pull over and let the brakes cool before starting the 5 - 10 minute test drive. Ideally you start with fairly cold brakes.
mike c.
