explain the hydraulic breaks
Within a hydraulic brake system, as the brake pedal is pressed, a pushrod exerts force on the piston(s) in the master cylinder causing fluid from the brake fluid reservoir to flow into a pressure chamber through a compensating port which results in an increase in the pressure of the entire hydraulic system. This forces fluid through the hydraulic lines toward one or more calipers where it acts upon one or two caliper pistons sealed by one or more seated O-rings which prevent the escape of any fluid from around the piston.
The brake caliper piston(s) then apply force to the brake pads. This causes them to be pushed against the spinning rotor, and the friction between the pads and the rotor causes a braking torque to be generated, slowing the vehicle. Heat generated from this friction is often dissipated through vents and channels in the rotor and through the pads themselves which are made of specialized heat-tolerant materials (kevlar, sintered glass, et al.).
Subsequent release of the brake pedal/ lever allows the spring(s) within the master cylinder assembly to return that assembly's piston(s) back into position. This relieves the hydraulic pressure on the caliper allowing the brake piston in the caliper assembly to slide back into its housing and the brake pads to release the rotor. Unless there is a leak somewhere in the system, at no point does any of the brake fluid enter or leave the system.