Do it yourself

Hi all, DH is on the road, and I can't reach him, so I am coming to you for help. Toilet is in the basement, runs to a sewer pit with a sewage sump pump, 2" pipe to check valve, 2" pipe to main sewer drain line which goes to septic. When the washing machine runs or the shower or anything that fills the pit and caused the sump to run, it sucks most of the water out of the toilet bowl. When the sump shuts off, we get a huge bubble of gas that comes out of the toilet and causes the water level to rise up. I am stumped, but am thinking there is something wrong with the check valve. In case anyone is wondering why we have a pit and pump, it's because the level of our basement is significantly lower than the main drainline to the septic. We connected to the old system. Thanks, Laurean