Good morning, I hope I can help you.
First of all you must establish if you are indeed losing coolant or if there is an airlock in the system causing the level to fall.
Wait until the engine is cold and then top up the coolant to the correct level. Next unscrew and remove completely the bleed screw. Squeeze the coolant hoses until you think all the air is out and then top up the coolant again if it has gone down.
Refit the bleed screw but only screw it in half way. Start the engine with the expansion cap also removed. Allow the engine to slowly warm up and at the same time squeeze he hoses again.
Observe the bleed screw and watch for air escaping. Once the air stops and it is a fresh stream of fluid escaping, tighten down the bleed screw. Again top up the coolant level, but this time take it just over the maximum line and refit expansion cap.
Run engine for another 10 minutes and then allow to cool naturally.
Recheck the coolant level when cold and top it up to the max level if required.
From this position you will be able to tell if you are indeed losing fluid as if it now goes under the max level then you must have a leak somewhere.
You can then pressure test the system to try and trace the leak.
My name is Bill and if you require further help after carrying out these checks, just ask.
ASE Certified Technician
City & Guilds