Gen 7 Toyota Celica Cooling issue.

[Raljix]

Hi Guys. About 3 weeks ago I noticed I was loosing vast amounts of coolant and upon looking in the engine bay I spotted a leak on my radiator. Paid 300 GBP at my local garage and had a new rad fitted with a new thermostat too. It has been 3 days since the new rad and ive noticed a drop in coolant (only a slight drop, no-where near what it was like before the new rad). The guys at the garage told me it was a nightmare to bleed and get all the air out. Why has the level dropped? is it because there may be some air still in the system and its just working its way out now? How long will it be before the levels settle? any in fo would be great. If someone knows if this has been covered in another post could someone link it....