[strictly]

okay, good. Just wanted to be sure, ive learned the hard way to go compression, ignition, fuel for diagnosis.

So what not just change the pump to the one with an external check valve fitted? Your pressure loss in 20 minutes, you have confirmed that was a CIS fuel pressure guage/tester on the system pressure side, or is that a guess?

Im sure you could "fit a check valve" but better to do it right, with the right pump. Sell the old one on ebay. What if you need to unfit that new check valve.