When we first started making Venus receivers the end caps had no end cap valves at all, and guys were having pop off issues that we could not really conquer.
So one of the customers rigged up an end cap with a valve and voila we have end caps with one valve that are best suited to 95% of the population.
A 2 Valve cap really helps during pumping, or when you have alot of air seepage between the penis and the liner.
Please remember that valves can corrode over time and cause that little ball not to seat well in the valve.
DO NOT wash your caps in really hot water or they become hard and a bit smaller.
For extended periods of time between use a small drop of 3 in 1 oil or wd-40 can keep the ball protected and lubricated and longer lasting.