So with the vacuum line disconnected and plugged, I set my FP to 58 psi, once I plug the vacuum line back in it drops to 52psi. But don't I want it to still be running at 58 when it's plugged in?

 

It is. Your engine has vacuum in the intake manifold at idle, looks to be about 6psi of vacuum.

The important thing is the pressure difference across the injector, so to keep it at 58 and you have 6psi sucking on the end, it only needs to push with 52psi on the top. Same reason why fuel pressure rises with boost.

 

Yup... what Atomic said.

Sounds like its all working properly!

 

So I'm ok then and it's just doing its job?

 

Give the throttle a blip. As you see 0 manifold pressure you should also see 58 fuel pressure. At the same time as you hit 10PSI you should also see 68PSI of fuel pressure.