Unfortunately, no drum stays round, and no rotor stays flat. Remember, a brake is a heat machine. It turns the energy of slowing a vehicle into heat, by way of friction. Heat, in turn, rearranges the molecules in metal, and warpage occurs, making proper brake adjustment impossible. The pulsing is generated as the linings or pads settle into a low spot in the drum or rotor, then are intermittently forced away by high spots as the wheel turns.
Brake fluid is intermittently compressed and released in the wheel cylinder or caliper.
The resulting pressure waves are transmitted back to the master cylinder and, ultimately, to the driver's foot. The fix is to machine or replace the drum or rotor. Remember, though, once a drum or rotor is machined, valuable heat-absorbing metal has been removed and warpage is likely to recur. A word about brake fluid Use only brake fluid that's approved for your vehicles. Approved brake fluid retains the correct viscosity throughout a wide range of temperatures.