how can i find the leak if the soil stack is failing the bailey air test

What readings have you achieved?

 

you can use smoke, or air pressure and soapy water on joints!!

 

Check the seals round your U gauge first.
38 mbar for 3 minutes if memory serves.
All your joints should still be exposed so process of elimination.
What did you use to lubricate the rubber seals when assembling everything up?

 

Ave a dump n follow ya nose