Most PPL trainers have hydraulic disc brakes on the main wheels, toe-operated via the upper part of the rudder pedals.

Standard configuration:

The brake system is typically installed only on the main gear — nose and tail wheels normally have no brakes.
Most brake systems act hydraulically on every main gear wheel — each wheel has its own master cylinder, brake line, caliper, and pads.

The pilot can brake left and right independently (differential braking), important for ground steering.

On small aircraft and motor gliders, the nose or tail wheel is usually controlled by the pedals, which are also connected to the rudder:

Pilot presses the left pedal → nose/tail wheel turns left + rudder deflects left.
Mechanical coupling via push-rods or springs.
Examples: Cessna 152/172 (nose wheel via rods), PA-28 (nose wheel via springs).
Exception: free-castering nose wheel (e.g. Cirrus SR22, DA40-NG) — nose wheel turns freely, ground steering only via differential brakes.

Component	Function
Master cylinder	Generates hydraulic pressure when the pedal is pressed
Brake line	Hydraulic fluid to the caliper
Caliper	Presses pads against the disc
Disc	Mounted on the wheel
Brake pads/linings	Wear items
Park brake	Mechanical or hydraulic hold of brake pressure

Phenomenon	Cause	Action
Brake fade	Disc overheating after long braking → reduced effect	Wait to cool; dose braking
Locked brakes on landing	Excess pedal pressure, wet surface	Release pedal, reapply; do not allow skid (flat-spots tyre)
Brake drag	Brake hangs on after release	Maintenance immediately — overheats tyre/wheel
Spongy pedal	Air in brake line	Bleed brakes (maintenance)
Hard pedal, no effect	No pad, master cylinder failure	Maintenance

Pedal firmness: hold pedals — pressure should hold.
On taxi just after start: brief light brake test at walking pace to verify function.
Visual: pad thickness adequate, no hydraulic leak, disc not scored.

On free-castering nose wheels (e.g. Cirrus SR22) ground steering is via differential braking.