Abstract: A modular check valve system for providing back pressure to a fluid flow having a housing with a seating end, a receiving end, a housing annulus, and a housing sealing surface. The modular check valve system can also have at least one cartridge insert fluidly connected to the housing. The cartridge insert can have a poppet engaging a cartridge housing sealing surface, a pair of O-rings positioned on the cartridge housing, and a rod secured to the poppet. The rod is configured to move the poppet from a sealed position to an open position. A spring guide with spring is disposed around the rod. A compression adjustment device is connected to the first rod opposite the spring guide to apply force to the rod sealing the poppet against the cartridge housing sealing surface and creating fluid back pressure in the housing annulus when the spring is decompressed.