repeating water pump leaks

On an '82 mercedes 300SD, the water pump was replaced before I bought the car last year. After I flushed the system it developed a very small leak from the water pump and the local mechanic said I needed a new
water pump.
It only leaked in the morning when the sun was warming up the car with the engine off and then it would stop leaking and the change in temperature stopped. Else when I started the car it leaked a little as the engine warmed up and then it stops leaking.
I got a new water pump and had about half the coolont replaced and after 1 month it is doing the same thing again. It seems like it is building up too much presuure in the engine block when the temperature increases with the engine off. Is there supposed to be a path for water back pressure in the engine block to go back into the radiator when the engine is just sitting there turned off and is getting heated just from the sun shining or air temperature increases?
Is it possible a small amount of citric acid flush is still in the system and eating up the water pumps? I flushed it for maybe 45 minutes because it was hard to get engine block coolant to circulate very much when doing a flush.
thanks, George
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don't over tighten the <fan> belts
the case, minus a few cans!
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I'll check this. I remember the mechanic had to adjust the belt tention along with the alternator's potition because the alternator's rotating part was hitting something. I think I don't have an original part number alternator. Maybe he made the belts too tight when he moved the alternator position to stop the hitting.
pool man wrote:

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The pressure cap controls the cooling system's pressure. If in doubt, buy a new cap.
The usual leak from a water pump is from its shaft bearing. Are you sure that's the source? It could be from the pump's mounting flange gasket or from a hose attached to its housing. Use a small mirror to trace the leak.
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Now it stopped leaking again but I'll look more closely the next time it does.
thanks, George
T.G. Lambach wrote:

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if i am not mistaken there are 2 belts as with 2 you don't have to tighten them up as much.
if you only have 1 check all your pulleys
some one may have refitted it with the wrong ALT.
the case, minus a few cans!
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