I have run an Accusump system both ways with and without the check valve. Before I built a good oil pan I had the low oil pressure light come on 5 or 6 times a lap and the Accusump saved the bearings WITHOUT the check valve, so I know it works without one. However, when I later had oiling issues unrelated to the check valve I put one in just to eliminate the possibility that it was contributing to the problem. So, the short version is, put one in for peace of mind if for no other reason.