Vented loop problem

[Spanki]

I have noticed that when I pump our Par toilet that a small amount of water comes up in the vented loop and sometimes a drop or two comes out through the duckbill in the vent. I purchased another vent cap with duck bill but still have the problem, it also makes a sucking sound. We replaced all the toilet hoses when I purchased Spanki (actually ALL hoses) and I made sure the hoses were routed correctlly as per the Catalina owners manual. Any one have a cure?:rolleyes: