With a bike featuring Shimano 486 Hydraulic Disc brakes and levers, what should I do if I wanted to maintenance them? I have no experience of brakes other than V-brakes so where do I begin? The front brake seem to work but the lever goes all the way down, the back brake doesn't work at all. The lever goes all the way down but the brake doesn't bite.
Hydraulic disc brakes pads are self adjusting for wear, so if the lever needs to be pulled back too far, or is hitting the handlebar the hydraulic system needs to be put through the proper bleeding process to remove air bubbles. You also need to check for leaks and loss of fluid, and check the wear on the brake pads.
Hydraulic fluid cannot be compressed, but air obviously can. When you pull the lever the air bubbles are compressing rather than the pistons and pads being forced towards the rotor. It's kind of like having a spring in the cable of a mechanical V-brake system.
If you are unfamiliar with hydraulic brakes, you may find it better to have a good bike repair shop do the bleed process - it requires a special kit, replacement fluid and has to be carried out following proper procedure. If you want to learn how to do it yourself there are video tutorials available online (here's one) but you really need to check the Shimano manual to make sure you are following the correct procedure.