If the bowl holds water
, it is not the bowl seal.
If you have new duckbill valves, it is not the bellows, it would not leak back to the vacuum chamber past the duck bill valves, and if the bellows leak, they will leak OUT when pumping "stuff" so you'll know...
First step is to tighten ALL the hose clamps between the toilet and the pump. Really tight. If any break, replace them with good ones.
If your duck bill valves were "years old" they were surely part of the problem. Check carefully the mounting of the duckbill valves and connections on the vacuum side of the pump.
Do NOT mess with the vacuum switch! If you start fussing with that, getting it to work
right again is very difficult. Just tighten the hose clamp securing it. Do NOT try to adjust it.