Try removing the relief valve entirely and see if you can get good flow. If you do, it means the relief valve is plugged up or stuck, or your primer pump can't develop enough pressure to overcome the 10-15PSI required to adequately lift the ball off its seat in the relief valve.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]