( Are you talking about a P-trap, or an actual valve that you can turn on and off? )
It is a swinging rubber flap that is supposed to let the shower water out and into the tank, but when any water tries to flow back the wrong way is swings shut and stops the flow. Here is one from amazon
http://www.amazon.com/Zoeller-30-0181-Check-Valve-Inch/dp/B0009WD1L4/ref=sr_1_fkmr2_2?ie=UTF8&qid=1436481045&sr=8-2-fkmr2&keywords=swing+check+valve+horizontal+PVC
they have it pictured in the vertical position but ours is horizontal.
Hope this helps.
