API POST method call return 503 when live

Hi, In my app I make a call to long running service (approx. 40sec). I set my request timeout to 60sec or even 90 sec. and I keep getting 503 when app is running live. When tested locally it works 100% of time. When live I can see the call is executed but after about 40 sec it returns 503 (backend service returns 200). When the same service returns faster the everything works ok. How to setup my app to respect timeout of say 60 or 90sec when running live? Perhaps there is another way of handling long-running tasks in ZenDesk apps?