Waste water being syphoned

[DerbyLiam]

Every time the bath is emptied the water in the shower trap is syphoned out.

External stack has a durgo valve on it and the bath and shower waste is teed together inside before going externally to soil stack

How could I get over this?

 

[Norcon]

Pipe them separately to the stack and remove the durgo as they are not designed for outside conditions.

 

[Whitespirit66]

A small air admittance valve on waste pipe between bath and shower should sort it. Increasing size of waste pipe between bath and stack may also help.