Unless the delivery valve discs in the pump are cracked up or the cylinder worn so much that the piston doesn't fit solidly anymore, there's not much to go wrong with the lift pumps. If it's pumping fuel and delivering pressure and isn't leaking, let 'er rip!

__________________
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]