W123 Brake Booster Failure

As our cars age, components will begin to fail and fail unexpectedly. 

While driving my 1981 240D several weeks back, I lost the assistance of the power brake system. Having never experienced losing power brake assist, I wasn't quite sure of just what had happened while attempting to slow the car from 65 mph while exiting the highway.

I did know one thing for sure, it took a lot of leg muscle and putting the car into neutral to come to a comfortable stop. Once at the traffic light, I noticed a loud hissing sound coming from under the driver side dashboard area. The hissing had to have been quite loud considering it was clearly heard over the clatter of the diesel engine.

Immediately pulling off onto a side street seemed like the best idea to find out just what might have gone wrong. The brake fluid reservoir was full and no leaks were visible at the wheels, which left an issue with the master cylinder, calipers, which seemed unlikely, or the brake booster. It wasn't until I put  my head in the footwell and pressed the brake peddle with my hand, did I realize it was definitely booster and vacuum related. That was confirmed yet again, when I arrived at my destination and could not turn off the car using the key. Thank goodness for the stop button Mercedes provided under the hood to burn your fingers on.

The following day I removed the booster and shipped it to Power Brake Exchange in California for inspection and a rebuild. They have rebuilt another booster for me with excellent success and great customer service. 

Several days later, Power Brake Exchange informed me that the "poppet valve" had torn, allowing vacuum to escape and causing the lack of brake assist. Who knew?

The following diagram from ATE shows where the poppet valve is located and it's function. This diagram also shows the general layout of the W123 booster whether your car has a Girlling or ATE system. 

The only purpose for this blog was to provide a bit of advanced warning that this part will probably begin to fail more frequently in more W123's as they age. The good side however, it can be fixed, and you can stop the car provided you give it some good leg. As I mentioned, shifting to neutral also made things much easier. 

Until next time - Drive safely!