The rad should always be full to just under the overflow pipe but don't use water, its
getting cold now weather wise and if you fill with water it will do two things, freeze up
and cause limescale build up inside the rad etc as well as not helping with corrosion.
Get some premix glycol ethylene, forlife or cool red from Toyota approx £20 for 5 litres, you'll
need 10 litres to fill an empty system.
Halfords also sell a pre mix
How old is the rad cap ? i'd replace that first, have you checked for coolant in the passenger
footwell, also what state is the rad in and coolant hoses, heater matrix feed pipes are prone
to fail when they get old.
To get air in the system usually means you have a leak somewhere or the expansion tank
is too low to let coolant be drawn back into the system or the rad caps failing