Here is the rule. Michigan is one of very few states that realize a check valve after the wellhead can cause negative pressure and possible contamination to enter underground water lines. The only way to have “positive pressure at all times” is to not have any check valves after the wellhead.
Rule 155. (1) The buried portion of a water service line between the well casing and the pressure tank in any installation shall be under positive pressure at all times. If a check valve is installed in the water line between the well casing and the pressure tank, the water line between the well casing and the check valve shall be in compliance with the requirements for a suction line pursuant to the provisions of R 325.1654.