It sounds like the threads on your check valve are rusted and that's causing the leak when your pump pressurizes the tank. Since the pipe looks old, the rust could have weakened the connection. To fix it, you may need to replace the check valve or the section of the pipe that's damaged. First, turn off the pump and drain the water. Then, remove the valve and inspect the threads to see if they’re salvageable or if replacement is necessary. If it's the pipe coming through the wall that's rusted, you might need to replace that part as well. It’s also a good idea to use pipe tape or thread sealant when installing new parts to prevent future leaks.