When I turn on the waterfall pump it does not always prime so I open the check valve and stick a hose and add water until it starts filling up and pushes water out then I screw the cap back on before full force o f the water pushes back. I then wait and open the valve slightly to let out air and do this a few times or maybe once until it builds enough pressure to prime.
I just learned that these types of valves also come with a spring and a plunger that pushes back on the water and closes the pipe until enough pressure is exerted on spring to let water flow.
I have only opened the two end valves ( there are a total of 4) and they don't have springs or the plunger. It is just the cap that you screw off partly to let air out or depending on sound and little water coming out I add water through the end to fill up the pipe and you can hear the prime happening. This is dangerous because I have had the water come out do fast I can't get cap on in time then I run to shut off the pump with flood water bring in dirt to the spillover tank for my pool - the water circulates from waterfall ponds to pool to spillover tank and then up to waterfall.
Could the lack of this equipment ]be the cause of my priming problems. There are three ponds with two lines each and the third pond has 4 which probably accounts for the 4 valves you see in the picture.