1991 Opel Astra Getting empty the refrigerant

Hi
Deaqr Gentlemen
My Car refrigerant(R134a) Getting Empty When The cooler works for 2 hours and much & I check all parts for leakage with some methods, there no leakage sign, however I replaced all tube with new one and replace expansion valve, it keep the gas when its work short time and when I going e.G travel I encountered this problem and this had me alot of payment.
Ihave a question and tha is : is there relationship betwin long time time working of A/C and Empting the gas?
please help me what must I do
Sincerely Mojtaba.