The pressure delivered to the injector is set by the pop pressure. The pump is capable of developing several thousand PSI of pressure if dead-ended.

I'd be extremely surprised if replacing fuel lines caused an IP failure. If you're curious, connect a known-good injector to a spare hard line and crank the engine over. Once the air is burped out of the line, you should see it firing the injector.

Being an OM606, be on the lookout for air ingress. There are several O-ring connections on various fittings that can leak air and cause all sorts of issues. Have the fuel filters been changed recently?

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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