Goodgenes,
you may have a break in the gasket between the TB and the line pipe(back end of SC). That gasket is two pieces of metal embedded in hard rubber and adapts the TB to the SC. It can separate at one of the metal plates and leak air. I know........ You have to be careful when you take the TB off and on so you dont torque that gasket and start it splitting....... Just another thing for you to check....