Mercedes engines since the mid-1950's used a breather pipe between the water pump housing and the cylinder head. The purpose was to allow air bubbles, that could gather in the top of the water pump housing, to be bleed off to the cylinder head. If air bubbles are allowed to accumulate in any portion of the water pump housing this prevents the suction effect of the water pump to take place. This cavitation causes loss of flow and can also cause wear. and damage to the water pump impeller and housing.* Because of the small inside diameter of the breather pipe and the banjo bolt holes, it easily becomes plugged by contaminants and corrosion. Once the pipe or the banjo bolts become plugged the air is not able to breath out of the water pump housing to the cylinder head and overheating can become a problem. I have documentation of a classic Mercedes with everything in the cooling system, and a newly rebuilt motor, that had an overheating problem that defied a fix. When I suggested replacing the breather pipe, the overheating problem was cured. I sell a complete kit with a new OEM German pipe, banjo bolts and the correct aluminum washers on eBay if needed. My eBay user name is mbpartsguy. *References: Cavitation in Hydraulic Turbines: Causes and Effects written by: naveenagrawal • edited by: Lamar Stonecypher • updated: 11/27/2 "The formation of vapor bubbles in cavitation is not a major problem in itself but the collapse of these bubbles generates pressure waves, which can be of very high frequencies, causing damage to the machinery. The bubbles collapsing near the machine surface are more damaging and cause erosion on the surfaces called as cavitation erosion. The collapses of smaller bubbles create higher frequency waves than larger bubbles. So, smaller bubbles are more detrimental to the hydraulic machines." "These bubbles coalesce with each other to form larger bubbles and eventually pockets of vapor. This disturbs the liquid flow and causes flow separation which reduces the machine performance sharply. Cavitation is an important factor to be considered while designing"