If it is a "used" drum it could have been involved in an impact of some sort. Again if it is a used drum, it may have been "turned" or "refaced" too many times, leaving it out of "true" and too thin. If the drum again is used, it may have been heated during continued stopping and then subjected to water on the road. The drum may simply "warp" thus giving you a pulsation or unequal braking
A leak in the braking circuit is a likely cause.. Brake fluid leaking from a poorly tightened joint etc - will cause the level in the reservoir to fall - causing the sensor to trip the warning light.
Check the brake switch located on your brake pedal shaft. May have to be replaced.
Air in a brake line or a bad master cylinder could cause this. Bleed the brake lines if you have been working on any brake cylinder. If this happened without any warning, the master brake cylinder is probably the cause and should be replaced.
If the brake pads come with shims or other anti-rattle hardware and it is not used on the vehicle, the brake pads will vibrate against the rotor as they are being applied and that will cause them to squeal.
The ground cable, on the battery may need to be tightened or cleaned. The alternator voltage regulator may need to be replaced.
The brake pedal has nothing to do with it. If air is in the system then bleed the brakes.
un equal adjustment of brakes will cause one brake to apply earlier and harder than other brake
your brake rotors are warped. You should repalce you brake pads and have the rotors resurfaced if possible, or replaced.
The BOO (Brake On/Off) Switch may need to be replaced. If it is bad, it would cause the brake lights to NOT illuminate when the brake pedal is applied.
There are many things that can cause: to leak from an automobile engine. The most common reason is a water hose that needs to be tightened or replaced.
Your brake pads might need to be replaced.
Rear brake cylinders or calibers are corroded and need to be replaced. Also drain the replace all brake fluid.