The brass piece is found on the right-side of the injection pump, the head of which looks like 15mm bolt with a smaller one inside. It has been over four years since I have done one so my recollection is a bit fuzzy. You might also post this question on the Diesel section of this forum.
I used to use Redline fuel additive regulary with fill-ups. Also Diesel Doctor or even Diesel Purge are good if you think you might have an algea build up.
Regarding the valves- they are supposed to be adjusted every 15k miles. From what you posted I don't see how long in miles but two years may be a long time. Diesel engines are very sensitive to a good valve adjustment.
Hope this helps, keep us posted,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)