You might have some object such as pieces of fuel hose stuck in the line restricting the flow. The fpr return line should not be restricted and allow any pressure except for the size of the line. It should be open all the way to the tank with no restriction.
It sounds like you have mistaken the pressure feed line for the return line.
You have a feed line from the pumps, under pressure, a single line with a tee to the rails, or two lines to the rails, under pressure, and the return line that only the size of the line is the restriction to the tank.
I guess with no vaccum to the fpr such as when you crank the motor the fpr is closed and the spring opens at the set pressure to return the fuel to the tank. You might put a hose on the return fitting into a bucket and see what the pressure goes to with the key on. That would tell you if the return line to the tank is where the restriction is.