If you put a 22 degree elbow, pointed upward on the main drain outlet (to prevent air entering from the outlet and flattening/slowing flow, it can create a siphon when the inlet is completely covered and no air enters. once air enters the inlet, the siphon will break, but until then, flow will be massive through the main drain in siphon mode.
What Rainman says is what you want. Its a upside down P trap that will let your pipe run full until the water level is down to a whirlpool will start and suck air breaking the seal.
These should not clog when the flow starts shes on.
![[Linked Image from corvettejunkie.com]](http://www.corvettejunkie.com/pb.gif)
