Air Conditioning problem
1991 Opel Astra 4 cyl Front Wheel Drive Manual 150000 miles

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.