This one will be really frustrating to bleed - the line is connected to the caliper above the bleed screw. You may want to unbolt the caliper and flip it over so air can be taken out of the calipers.
You’ll want to buy or borrow a tower bleeder - it’s a bottle-on-a-stick that allows fluid to be put into the system with enough pressure head to make it through to the back of the kart readily.
TaG karts heat the rotor heavily enough to want good fluid (Castrol SRF if you don’t like to bleed it regularly, any fluid with a dry boiling point over 550F if you’ll bleed it before every big race). If it’s a 4-cycle or Yamaha Can you can just use Prestone DOT3. Before you fill the system with DOT3/DOT4, take a little sample of the fluid and mix it with a little sample of DOT3/DOT4 and make sure it’s miscible and not DOT5.
If your brakes leak down over the winter then they leak, period, and you should fix the leak before taking the kart out on track. Clean the brake system off then use a c-clamp around the brake pedal and front bumper to apply the brakes all night to find where it is leaking from.