Pump through functionality in subsea valves using external manifold

A subsea system for use at a well, the subsea system comprising:

• a subsea valve system for use in a subsea landing string, the subsea valve system comprising:
• a subsea valve;
• and
• an external manifold in fluid communication with the subsea valve, wherein the external manifold comprises a cylinder, a split manifold piston within the cylinder, 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, wherein the split manifold piston is configured to move within the cylinder to isolate the above bore inlet from the open chamber outlet to block pump through functionality while a respective force exerted on the split manifold piston by the above bore pressure is less than or equal to a respective force exerted on the split manifold piston by the below bore pressure and to fluidly couple the above bore inlet to the open chamber outlet to enable pump through functionality while the respective force exerted on the split manifold piston by the above bore pressure is greater than the respective force exerted on the split manifold piston by the below bore pressure.