Honda FourTrax Rancher TRX420FA5 2019 Owner's Manual page 175

Brakes
Front Brake Fluid Level
RIGHT HANDLEBAR
(1)
(1) LOWER level mark
With the ATV on a level surface, check the fluid level.
It should be above the LOWER level mark (1). If the level is at or
below the LOWER level mark, check the brake pads for wear
(page 167).
Worn brake pads should be replaced. If the pads are not worn, have
your brake system inspected for leaks.
Servicing Your Honda 165