A subsea valve system for use in a subsea landing string includes a subsea valve and an external manifold in fluid communication with the subsea valve. The external manifold includes a cylinder, a split manifold piston, an above bore inlet that is configured to be exposed to an above bore pressure, a below bore inlet that is configured to be exposed to a below bore pressure, and an open chamber outlet. The split manifold piston is configured to move within the cylinder to fluidly couple the above bore inlet to the open chamber outlet to enable pump through functionality while a respective force exerted on the manifold piston by the above bore pressure is greater than a respective force exerted on the manifold piston by the below bore pressure.