Checking rail pressure while cranking

[Konstantin]

(11-10-2019 )tree98 Wrote:  Does it only do this after it has sat for a period of time? Could be the check valve is bad and bleeding off the prime.

sometimes yes, but sometimes it does it for no reason when engine is still. especially if i try to crank it right away, without giving it 10-15 seconds ignition time. how do i check check valve?