I had a Rialta for several years and I knew there were some long threads on the RialtaTech group on this topic like this thread: https://groups.io/g/Rialta-Tech/topic/2327489#10753
There are also some resources on the Baldy site, rialtainfo.com.
I cant think of a reason why your hose seems to have a one way plug, unless maybe thereís a midstream filter thatís plugged. But when you do get past that everyone agrees you have to prime the pump with water after having everything apart.
In our case I recall I eventually gave up on having a working shower drain, disconnected the drain and just let the shower drain onto the ground. Not an eco-friendly solution, but we used the shower very rarely.