Is there only one check valve on this machine to prevent siphoning from customers drain back into the sump? I was call here to service the dishwasher for a no pump out situation and cleared white cholesterol type build up from the drain hose. At the pump outlet port, I removed the check valve to view it for an obstruction and found none. Dishwasher pumps out ok, but water comes into same from customers sink. Drain hose is not on Y tail but rater a T with no ck valve in the drain. I was told there may be a ck valve missing inside the dishwasher drain hose? Please advise. PS the drain hose is looped at top of cabinet from drain then back down to pump. Thanks. :)