Some suggestions: 
 
- as mentioned try using a hard line all the way down to the sump 
- using a sanitary T (I know, they're hard to find in PVC) would help 
- tap some threads into the hole at the top and screw in a JG ball valve. 
 
I think the final suggestion would have the most effect because a Durso is all about controlling the amount of air that gets mixed with water.  With the ball valve you'll be able to do that.  I remember back in the day (waay back) when Wendell at OA first showed me how his Dursos worked with ball valves controlling air intake. 
		
	
		
		
		
		
		
	
		
		
	
	
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