Try clamping the rubber parts of the fuel supply pipe and fule return pipe, one at a time. That will tell you which way the pressure is being lost. If neither make any difference (ie you still get a loss) it points to leaking injectors or an external (pipe) leak. Not much fuel needs to leak to reduce the pressure. Also join in a gauge if possible to watch the pressure go down.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i