1988 Buick Regal fuel delivery

Probably not the regulator, if pressure is good at idle and drops under load, it might be a fuel pump issue. To check the injectors will require their removal, however unless there is a driveability issue, I would save that task for last. The check vale is integral to the pump and cannot be individually changed. Also inspect the filter for blockage or for a crushed or restricted fuel line.

Hello, you might clamp the supply and return line, after pressure is built, see if it holds.

I don't know how you cycled the fuel pump, there should be a prime connector, close to the battery, you can use that circuit to cycle the pump. No need to use the key. I have also used a fuel pressure gage in place of the fuel filter, as far as checking drain-back through the pump, that way I can clamp the hose to the gage and not worry about a plastic insert in a fuel line hose. Some hoses may have a plastic insert, some don't.