I installed a guage at the fuel rail some time ago using parts acquired from a hydralic fitting supply company. It took a bit of digging around to get the correct parts. Whatever kit you try to order it must have the correct fittings to connect to the supply line and fuel rail or it will be useless. The official Volvo dealer type tool is most likely sold at a terribly inflated outrages price so forget that. As it turned out my pressure was normal so it was not a very productive diagnostic. In my opinion if both pumps are running, fuel filter not too old (actually I think that giant filter never clogs) then the fuel pressure reg is the prime suspect. Even then I doubt that because normally when bad they leak into the vacume line rather than cause wrong pressure.

Bleed down of the fuel pressure is normal after the car sits for a while and will not cause delayed start as pressure builds instantly once the pumps start. I know this from observing my guage when I had it hooked up.

There are many other possible causes of low power and delayed start. What about the basics such as plugs, wires, distributor, rotor OK? Fuel pump and radio supression relays already resoldered?

What year model do you have?????