You should have NO bubbles in the fuel system, ever. Bubbles indicate air ingress, and air ingress means running problems.

There was a thread on this forum within the last year where the OP was tracking down a mystery air leak that turned out to be the fuel heat exchanger was cracked just enough to leak air but not fuel.

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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]