Fuel accumulator bench test

A vacuum test, 20 inches should hold. You are checking for the integrity of a rubber internal diaphragm. You can also use the same tool for brake bleeding.

https://www.amazon.com/HFS-Brake-Bleeder-Vacuum-Tuner/dp/B00NP60URE/ref=sr_1_5?s=automotive&ie=UTF8&qid=1525449939&sr=1-5&keywords=vacuum+test

 

Functionality failures of these components are so rare that its anecdotal...on the other hand, the more "normal" failure is that the diaphragm(see other posters test procedure)gives up the ghost, and they leak...
Now, the car has a nearly "un-findable" gas odor "leak"-it will REEK of fuel when it gets bad...the metal vent tubes, depending upon how the individual unit has been manufactured, can actually be pointed "upwards" which REALLY can make it difficult to find....In most cases, it is rather horizontal to downward oriented, and as such, will leave a nice puddle of gas to find. To complicate matters, these are normally mounted with, or adjacent to, the fuel filter(s) and pump(s), model and configuration dependent.
Amazingly, with such a leak, the performance of the car- 8 out of 10 times- will not be the complaint: But will be-"it stinks", and "there is a fuel leak stain under the middle to rear of the car on my garage floor...".
Good find, these are NOT cheap, and almost IMPOSSIBLE to find-Bosch stopped making these in the 2002-2006 range....getting rare....

 

I would block off one end, apply fuel pressure (attach a fuel pump) to the other end, then see if there is any fuel leaking out of the fuel tank return port. That's how they usually fail.

 

They are widely available but there are some variations of different models. eg. My Swiss model Mondial 3.2 (which the accumulator is supposed to cross to the 400i as well) had a different Bosch part number and only a single inlet connector, that crosses to Mercedes. https://www.ebay.com/itm/Mercedes-R107-W116-W123-280E-Fuel-Injection-Fuel-Accumulator-Bosch-0438170004-/371510260386.

Maybe some variants are getting hard to source.