View Full Version : No brake boost assist

12-18-2013, 08:41 PM
Master cylinder failed on one of our '08 chassis cabs and leaked fluid into the booster and vacuum tank. Changed all 3 parts with factory new stuff and now I have no brake boost. Checked the lines to the vacuum pump and all are good. Also swapped in a spare vacuum pump I had to no avail. What is the vacuum reading supposed to be?

This truck has high egt codes which I plan on taking in for a regen. Also, it has codes for MAF range performance. No ABS codes stored, but also MAP sensor, IAT, UO122 (lost comm with Vehicle Dynamics Control Module), UO103 (lost comm with gear shift module), UO101 (lost comm with TCM), and finally, PO340 (cam pos sensor A circuit, bank 1 or single circuit).

So whatcha got for that?! Truck starts and runs, a bit of a dog in acceleration, and of course no brake boost. Traction control light and ESP lights are on.

12-18-2013, 08:51 PM
I have not had the master cylinder off a 906, but most every MB has an o-ring to seal the master cylinder to the booster. If you have a vacuum leak there it will never have assist.

12-18-2013, 09:11 PM
Sometimes, a second set of eyes would be nice to have. Thank you. For some reason there is no o-ring on the new master cylinder. Stand by for results...

12-19-2013, 12:12 AM
All is good

12-19-2013, 12:07 PM
can you elaborate?

12-19-2013, 04:41 PM
Some parts are hit and miss when you order them, meaning, you'd THINK they would throw a $.10 o-ring on the back of a $200+ master cylinder.

Your vacuum starts at the vacuum pump, located directly on the front of the passenger side head. That hard plastic line from it snakes across the top of the motor over to the booster. At the booster, the line t's off down to the vacuum reservoir down to the driver's side nose of the vehicle. The reservoir can be related to a "capacitor" for reserve vacuum. Back up at the booster, there is a check valve at about the 5 o'clock position. The check valve keeps the vacuum inside the booster (only one way in, not out). There is about a 2" o-ring that fits around the backside cylinder of the master cylinder where it mounts inside the booster. If that o-ring isn't there or has failed, the vacuum that comes in through the check valve will be lost through the backside of the master cylinder.

12-19-2013, 11:07 PM
I do not think MB sells anything for .10 :lol::dripsarcasm:

12-20-2013, 06:39 AM
You're right, I've paid up to nearly $20 for the green o-ring that goes from the plumbing to the turbo itself. The butterfly valve o-ring on the coupling is about $10 if I remember correctly.