this is kinda random but i'm just trying to gather more information.
why is it ok to run vacuum lines on a turbo car to a source that ends up seeing positive pressure(mainly the fpr)? you would think that you would want it to stay in vacuum becasue that is the way it was origionally designed. i know that it doesnt see boost all the time, but during the times it does you would think that it wouldnt function correctly.

I thought about this same thing myself - read the sticky on how the fuel system works.

consider how the stock FPR works - normally open - meaning most fuel is dumping back to the tank.

Full vacuum - no throttle - is pulling the actuator or valve open - allowing more return flow and lowering fuel pressure at the rail.

Less or little to 0 vacuum - full throttle - actuator or diaphragm cannot fight the spring pressure- FPR is now limiting flow return - increasing pressure to the rail.

Boost - full throttle - just pressurizing the diaphragm but the actuator is already full open slammed against the stop by the internal spring = no additional change in flow characteristics - hence the necessity for an FMU or injector size/duty cycle change to add fuel for boost.
Since the FMU works opposite of the stock FPR (still normally open type of valve) but the pressure acts on the opposite side of the valve - pushing the valve closed in response to boost and further raising the fuel pressure.

If you used a check-valve on your FPR - it would only allow the same fuel pressure at idle all the time - starving your engine.

I do not state to be an expert about the stock FPR - I have not cut one open to get all the details, but the above makes sense to me.

I'm interested in how boost to the Stock FPR may affect the life or strength of the valve - as I assume it has a diaphragm has some limit, but consider the fuel pressure on the other side of it - boost pressure is minor.