Not sure about the FPR thing (never experienced one), but I guess if the FPR sends the gas back in to the tank, the pressure will be low at the valve. The valve is just at the end of the fuel line (i.e. under the beauty cover front bank, driver side, cylinder 8 - top right from the injector). The spray test will just tell if you have gas, how good is the pressure … not very accurate (you got to have an eye for it something like … yeah looks like is about right). The best analogy is trying to measure the tire pressure by the noise the air makes when you push in the tire valve.
Here are some pictures of the two fuel lines available on ’97 STS:
Stainless steel (I hope you have this one) Nylon (change-it as soon as you can, is a recall for it).
The service valve is (looking to the first picture) in the right side of the picture (about 1/4 from the bottom), looks like a tire valve just bigger. As for the second picture is about in the same location.
Here is a picture with both fuel lines removed from the car, the easiest to spot valve is on the metal one (the thing in the tip).