WC water level drops with new set-up

[Ged Hunt]

I had en-suite installed in a bedroom, but because of problems linking to sewer, waste from WC/shower/basin runs into a new soil pipe at side of house, which goes into a tank. When level in tank rises, a pump in the tank sends it all up a small pipe, which goes around to back of house and feeds into original soil pipe. Two feet below that junction the soil pipe from another bathroom feeds into the main soil pipe (see pic). Whenever the pump kicks in to clear the tank, the water level in bowl in the other WC falls drastically. We flush it when we've heard pump and it restores water in bowl. But if we fail to do that for day or two it won't flush. Any thoughts? Vacuum? Suction?

 

[rpm]

Pump is creating a vacuum. Put an AAV at the highest point on that 50mm(?) pipe.