Mine came without also but one was added by the PO who failed to keep his duck bill valves in good shape for the sealand diaphram pump, it decided to syphon and lucky for him he noticed his holding tank filling . Even the closed loop that was added has a vent valve that has to be taken off and cleaned under hot water to make sure it does not plug and cause syphon problems. I pull the vent valve out each spring and flush it out good.
I just ordered 2 new valves for my sealand macerator, the vent valve will be cleaned during the change out also. Just not worth the risk to me since my seacock is left open