E30 losing fuel pressure


hi Guys, I got a quite particular problem I can’t seem to solve, I own a bone stock E30 318I m10 '78 (pride and joy) and its got a fuel problem. My car wont start… its not the ignition (runs fine on brake cleaner) . so we checked the fuel system. Fuel is getting to the rail, And the pump/ filter are new and working. When we measured the fuel pressure its getting to 3bar (normal) whilst cranking. but after it won’t hold pressure… my car being a 78 has only one fuel pump with a valve next to it ( near the driver side rear wheel). could this valve be the problem(leaking back into the fuel tank?. furthermore we manually put a vacuum on the FPR and also clamped the return hose to check is that helped( to raise the pressure on the rail…didn’t seem to do much… the fuel does seems to run out as fast as it comes in the rail, does this mean the FPR is broken and wont hold fuel in the rail?.. what do you guys think?

thank you for taking the time, excuse me if my english reads somewhat incoherent, it’s my second language (I’m from holland) .