Hi these do as name suggests store hydraulic fluid for when the braking system has a major failure. they apply a great deal of force to stop the truck should the hydraulic pump fail on the engine so they have a safety purpose. so my guess is these are failing or have failed due to pressure or rust.

Hello

One will be sat on top of the Brake Master cylinder, It will have 4 pipes coming away from it and may have an electric connection on top

The other will normally be for the Clutch

Andy C

FORWARD???? You may need your bearings.

Try the "what part is this" thread on the peach parts or benzworld websites. The merc owners, techs, parts guys, and experts are all reading it and answering because it seems every car they make is different from the others.

The answers you get on there can be really in depth and come from experienced, and well meaning people.

rear axle fluid reservoir bottle. ,although this can be found on the automatic petrol version W123 series