Hi folks,

I will post this in the ML section also, but here is the story. My wife's 2012 ML350 Bluetec has been having some sporadic brake issues, and today it turned on the brake warning light. I put on my Xentry reader and it came up with the code for a bad brake vacuum pressure sensor. The problem is this, the sensor is on the bottom face of the brake booster, under the master cylinder. I have fairly long fingers, but it is almost impossible to get to the sensor with the master cylinder bolted to the brake booster.

My question is this.....if I unbolt the master cylinder from the booster without opening any brake lines, it should not require bleeding the brakes, correct? It has been many years since I have had to replace a master cylinder but if I remember correctly, as long as the brake lines are not opened or removed, it should be a simple unbolt, then bolt back up procedure.

Many thanks

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

Normally that is the case.

Since you have the Xentry you have the WIS which will tell you exactly what you need to do. Since these are semi active brakes (for things like the HOLD function, the rotor dry in the rain function, pre-safe brake, etc) it would be good to check with the WIS for the procedure on the booster.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech