I had an S600 doing the same thing but only on the right front side. It would slowly raise up over a few minutes then suddenly drop and kept repeating this. For mine, flushing the system and replacing the front valve block fixed it, also replaced the rear valve block for another problem. They should know to bleed the system and it should have been done after the repair. Check your the RO with your reciept to see if that was listed. I'm not too sure what else might be causing it for you but it wouldn't be out of the question for the new part to be faulty. Take it back to them and let them fix it right, as long as its under warranty, nows the time to get it fixed.

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350