If you do see bubbles in the fuel lines replace all of the clear plastic ones. They get brittle from the heat and age, if someone bumps or touches them they can leak.

This makes sense since they probably were playing with them when they replaced the head and didn't change them.

This is very commen, poor design on MB's part. But at least they are cheap to replace.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200