Most common cause is imperfection in rotor or rotor's. Rust or hot spots.
Contaminated fluid? Frozen calipers?
the rear brakes are grabbing. the linings may have fluid on them from wheel cylinder leak or dust buildup. sanding the shoes may help .
Applying the brakes.
Yes, applying the brakes on a bicycle should slow it down. Little squares of rubber are pressed to the metal rimming on the bicylce's wheel when you pull a handle brake, or pedal backwards (depends on if it's an adult bike or children's bike).
Using brakes slows your speed.
brakes, or no sun
Yes, that will work.
faulty master brake system
probably not your engine, sounds like its time for new brake pads. if this doesnt correct it have the master brake cylinder checked
If you suspect something, get them checked. If you get vibration while applying the brakes, get them checked. If you get grinding noise from the brakes, get them checked.
Brake rotors or drums need to be machined or replaced. If it causes steering wheel shaking, check front brakes. If happens when applying parking brake look to rear brake problem.
lightly tap your brakes