I'm bench testing two fuel pressure regulators (PN 0 280 160 200).
One unit regulates line pressure to 40psi while the other regulates to 39psi.
Both reduce line pressure to approx. 32psi at 0.5 bar. Here's my question,
The 40psi unit allows line pressure to bleed off almost immediately, while the other holds pressure at approx 35psi almost indefinitely. What gives? I thought line pressure was held by a check valve in the pump and has nothing to do with the regulator. My test setup shows otherwise.

Also does the 1psi (2.5%) line pressure really mean anything to performance?

Any thoughts?

0.5bar=7.25PSI so the difference seems correct, assuming you're applying vacuum not pressure.

AFAIK, those are L-Jet regulators that should work at 30PSI (2bar) with no vacuum.

I have never used that type of regulator but...

Almost all regulators mount on the retrun line. The injectors pressurized by the pump and the requlator holds the pressure. Your pump dead heads around 45-50psi. the regulator releives this pressure down to 40psi.

I would use the one that holds pressure first.

That's the one I chose.

I double checked.
The L-Jet pressure regulator must hold exactly 35PSI without vacuum not connected.