I’m working on a friend’s 1957 Johnson 18 hp. I verified that the motor is not supplying air to pressurize the gas tank by using my tank which is in good working order. The motor will run as long as I occasionally prime the carb using the button on the tank. After a few minutes of running it like this there is no pressure in the tank when I crack the gas cap. Is there a check valve or something else on the motor causing this issue?