Just an FYI for anyone who might be experiencing similiar problems.. I have a 1989 760 Turbo/IC.. The car began to run poorly with poor power and the main fuel pump began making noise.. To make a long story short, after changing the fuel filter (needed one), the primer pump and the main pump (probably on its last leg anyway), the problem turned out to be the Fuel Pressure Regulator..
Before doing any of the other work, I had pulled the return off and fastened a short piece of hose to it and lead to a gas can. Nothing came out.. Well after confering with a Volvo "mechanic", he said this meant nothing and that I probably just didn't have enough Fuel Pressure.. Went ahead and changed the primer, fuel filter and fuel pump and noise, poor running continued.
So, I quit listening to anyone else, checked the plugs (they were carbon fouled) and checked this site. Well, I went down to the local Volvo dealer and purchased a Volvo FPR for less than the local parts houses wanted.. Got home, put in on and now had fuel going thru the return. I then changed the plugs and she seems to be running well... Hope someone else can benefit from this...
I guess the good news is I have an entirely new fuel system.. Which I'm sure I needed as the FPR, primer and main pump seemed to be OEM..
Regards..