18 x 30 basin w/1/2 hp pump top of lid to waste line connection is about 4ft.,there is a toilet,shower,basin,and washer all tied into a 2in vent line that runs all the way to the roof,the toilet is the closest fixture to the basin about 4 ft away.Everything drains very quickly and the toilet flushes well.My problem is that when the pump cycles (only 5 seconds) the water in the toilet moves and this evevtually lowers the water level in the toilet.How do I fix this? Is the pump to powerful, the vent to small? Someone told me to drill a 3/16 hole 2 inches above the pump discharge inside the basin to offset the suction created by the pump. Thanks for your help ,great forum and website.