Wheel brakes are normally used to slow the aircraft down during landing roll on the runway and to aid in directional control during ground handling operations as taxiing, steering and parking.
In small GA aircraft the brakes are connected to the main landing gear only and are usually operated independently from each other.
The brake systems are operated by hydraulic pressure and the rudder pedals have the brakes installed on top and are toe operated. Some aircraft use heel brakes or even with separate levers in the cockpit operated by hand (e.g. Pipistrel, DynAero MCR and Tecnam).
With larger aircraft the weight increases but the force pilots can apply will usually be about the same for every human being. It is therefore important that the pilot gets help in the form of brake boosters or power assisted brakes to handle the heavier aircraft.
The wheel brakes are usually made of the disc type, but on some exotic aircraft the drum type with two brake shoes inside is still being used. You know them from old ancient cars too.
With this model the two brake shoes are operated by either pneumatic (air) or hydraulic (oil) pressure or maybe even manually with a cable (very rare). You won't see these on modern aircraft anymore. Inside there are a couple of springs attached to the shoes to make sure that the brake contacts the drum evenly as friction will try to move them slightly. The springs also make sure that the shoes retracts when brake pressure is released.
Very popular as they are lightweight and the disc sits between the braking pads clamping them when brake pressure is applied. For heavier aircraft multiple brake calipers and or multiple discs can be used to increase braking capacity.
The brake disc is made from steel and bolted onto the wheel and rotates with it. The clamping part, caliper, contains two brake pads and is self centering. When brakes are applied the oil pressure moves a piston cylinder arrangement inside the caliper and the pads will clamp the disc. Resulting in an even pressure on both sides.
Most light aircraft have two independent brake systems on the upper part of the rudder pedal. This part is hinged and connected to a master cylinder. High pressure tubing is used to connect the master cylinder to the brake cylinder in the caliper. Special hydraulic fluid (DOT4, AeroShell Fluid 41) is used to transmit the brake pressure. Careful: this fluid will eat away paint so keep it well clear from other surfaces.
Make sure that at the start of the takeoff run you place your feet low enough as not to ride the brakes as this would result in a sluggish takeoff roll and very hot, even burnt brakes.
As the aircraft has two separate brake systems the pilot can use differential braking to help steer the aircraft, some aircraft (Pelican PL, Cessna) have their nose wheel steering connected to the brake system where the first 10° left or right the nose wheel moves and with further rudder movement the main gear will help steer the aircraft, helpful during taxi and parking.
Some aircraft have a sort of ratchet type system where they keep the brake pressure in the system, holding the aircraft in its place. Others use a hydraulic valve and after pressing the toe brakes they close the parking valve, thereby locking the hydraulic pressure. Check the POH to see which type of system is installed in yours.
The simplest system is the boosted brake. In this systems main hydraulic pressure is used progressively through a valve to help the pilot applying the brakes.
With power assisted brakes the brake system is fed by the main hydraulic system with a much higher pressure than the pilot can apply. Brake pressure regulates main hydraulic pressure to the wheel and caliper.
These power assisted brake systems usually have a backup in case the main hydraulic pressure fails so the aircraft can still be stopped by using the brakes.