Yes, the check valve is required to stop the pipe emptying. It also helps keep the pump primed. Your problem may have been a lack of suction from the pump not opening the check valve, so check the pump impellor. If it hasn’t been used for a long time, either replace the impellor or take it out and lubricate it. I use a small amount of Vaselene, which helps keep the blades flexible and helps seal around the pump face.
The membership association for owners of Dufour Yachts